Update projectFromJSON & upgrade function signatures (#1518)

This creates better flow type coverage for the upgrading
from older Project types feature.

Note projectFromJSON's function signature changes like
this:

- (Compatible<Project>) => Project
+ (Compatible<any>) => Project

And that makes sense, because we use this function to
validate an object we parsed from JSON at runtime. It
could actually be anything.

Added benefit is that is makes writing unit tests possible.
Because now will flow not throw a type error when we provide
something other than Compatible<Project> as input, to test
upgrading or validation functionality.

Note that the underlying utility fromCompat already uses
Compatible<any> for the same object.

src/core/project.js

@@ -42,7 +42,7 @@ export function projectToJSON(p: Project): ProjectJSON {
   return toCompat(COMPAT_INFO, p);
 }
 
-export function projectFromJSON(j: ProjectJSON): Project {
+export function projectFromJSON(j: Compatible<any>): Project {
   return fromCompat(COMPAT_INFO, j, upgrades);
 }
 
@@ -54,7 +54,7 @@ export function encodeProjectId(id: ProjectId): string {
   return base64url.encode(id);
 }
 
-const upgradeFrom030 = (p) => ({
+const upgradeFrom030 = (p: Project_v030 | Project_v031): Project_v040 => ({
   ...p,
   discourseServer:
     p.discourseServer != null ? {serverUrl: p.discourseServer.serverUrl} : null,