Fix 404s in timeline and legacy-mode interlinks (#1305)

Summary:
All links in SourceCred must use the `Link` component, providing either
an external URL `href={…}` or an internal route `to={…}`. Any uses of a
raw `<a>` element for internal routes will incur 404s when the
application is hosted on a non-root path, as is currently the case on
the production website.

The change to `FileUploader` is not strictly necessary, as the link has
no styled text and uses a `data:` URL, but there’s no reason not to.

Fixes #1304.

Test Plan:
Build the static site:

```
scripts/build_static_site.sh --target cred --project sourcecred/example-github
```

Then run `python3 -m http.server` from the repository root directory—not
the `cred/` subdirectory—and navigate to the timeline cred view:
<http://localhost:8000/cred/timeline/sourcecred/example-github/>

Observe that the “(legacy)” link now has the correct styling and
correctly navigates to the legacy mode page when clicked: prior to this
change, it would navigate to a URL without the proper `/cred/` path
prefix, yielding a 404. On the legacy page, verify that the “timeline
mode” link has the same properties.

Then, visit <http://localhost:8000/cred/test/FileUploader/> and verify
that the inspection test still passes.

Added a regression test to catch further such errors. Note that
reverting the code changes in this commit causes the test to fail, and
that running it with `--verbose` prints the problematic files.

wchargin-branch: fix-bad-routing-404s

sharness/test_no_raw_anchor_elements.t

@@ -0,0 +1,32 @@
+#!/bin/sh
+# shellcheck disable=SC2016
+#
+# Components must use the `Link` component from `webutil/Link.js` rather
+# than raw `<a>` elements. The `Link` component properly handles
+# client-side routing, as well as providing consistent styles.
+#
+# See <https://github.com/sourcecred/sourcecred/pull/1305> for an
+# example of how to fix this error.
+
+export GIT_CONFIG_NOSYSTEM=1
+export GIT_ATTR_NOSYSTEM=1
+
+# shellcheck disable=SC2034
+test_description='check that bare <a> elements are never directly used'
+
+# shellcheck disable=SC1091
+. ./sharness.sh
+
+# shellcheck disable=SC1004
+test_expect_success "application components must use <Link> instead of <a>" '
+    test_must_fail git grep -nF "</a>" \
+        ":/src/*.js" \
+        ":(exclude,top)*/__snapshots__/*" \
+        ":(exclude,top)*/snapshots/*" \
+        ":(exclude,top)src/webutil/Link.js" \
+        ;
+'
+
+test_done
+
+# vim: ft=sh

src/explorer/TimelineExplorer.js

@@ -10,6 +10,7 @@ import {
   type TimelineCredParameters,
 } from "../analysis/timeline/timelineCred";
 import {TimelineCredView} from "./TimelineCredView";
+import Link from "../webutil/Link";
 import {WeightConfig} from "./weights/WeightConfig";
 import {WeightsFileManager} from "./weights/WeightsFileManager";
 import {type NodeType} from "../analysis/types";
@@ -121,7 +122,7 @@ export class TimelineExplorer extends React.Component<Props, State> {
         <div style={{marginTop: 30, display: "flex"}}>
           <span style={{paddingLeft: 30}}>
             cred for {this.props.projectId}
-            <a href={`/prototype/${this.props.projectId}/`}>(legacy)</a>
+            <Link to={`/prototype/${this.props.projectId}/`}>(legacy)</Link>
           </span>
           <span style={{flexGrow: 1}} />
           {this.renderFilterSelect()}

src/explorer/legacy/App.js

@@ -146,7 +146,7 @@ export function createApp(
           <h2>SourceCred Legacy Mode</h2>
           <p>
             Back to{" "}
-            <a href={`/timeline/${this.props.projectId}/`}>timeline mode</a>
+            <Link to={`/timeline/${this.props.projectId}/`}>timeline mode</Link>
           </p>
 
           <ProjectDetail projectId={this.props.projectId} />

src/explorer/weights/WeightsFileManager.js

@@ -3,6 +3,7 @@
 import stringify from "json-stable-stringify";
 import React from "react";
 import {FileUploader} from "../../util/FileUploader";
+import Link from "../../webutil/Link";
 import {MdFileDownload, MdFileUpload} from "react-icons/md";
 import {type Weights, toJSON, fromJSON} from "../../analysis/weights";
 
@@ -16,14 +17,14 @@ export class WeightsFileManager extends React.Component<Props> {
     const onUpload = (json) => this.props.onWeightsChange(fromJSON(json));
     return (
       <div>
-        <a
+        <Link
           download="weights.json"
           title="Download your weights.json"
           href={`data:text/json,${weightsJSON}`}
           style={{color: "black"}}
         >
           <MdFileDownload style={{margin: "2px"}} />
-        </a>
+        </Link>
         <FileUploader title="Upload weights.json" onUpload={onUpload}>
           <MdFileUpload style={{margin: "2px"}} />
         </FileUploader>