Allow to generate lower case header references through the config (#1305)

This makes the references consistent/compatible with GitHub,
GitLab, Pandoc and many other tools.

This behavior can be enabled in config.json with:

```
"linkifyHeaderStyle": "gfm"
```

Signed-off-by: hoijui <hoijui.quaero@gmail.com>

config.json.example

@@ -127,6 +127,7 @@
         "plantuml":
         {
           "server": "https://www.plantuml.com/plantuml"
-        }
+        },
+        "linkifyHeaderStyle": "gfm"
     }
 }

lib/config/default.js

@@ -162,5 +162,6 @@ module.exports = {
   allowEmailRegister: true,
   allowGravatar: true,
   allowPDFExport: true,
-  openID: false
+  openID: false,
+  linkifyHeaderStyle: 'keep-case'
 }

lib/web/statusRouter.js

@@ -99,7 +99,8 @@ statusRouter.get('/config', function (req, res) {
     version: config.fullversion,
     plantumlServer: config.plantuml.server,
     DROPBOX_APP_KEY: config.dropbox.appKey,
-    allowedUploadMimeTypes: config.allowedUploadMimeTypes
+    allowedUploadMimeTypes: config.allowedUploadMimeTypes,
+    linkifyHeaderStyle: config.linkifyHeaderStyle
   }
   res.set({
     'Cache-Control': 'private', // only cache by client

public/js/extra.js

@@ -867,7 +867,11 @@ const linkifyAnchors = (level, containingElement) => {
     if (header.getElementsByClassName('anchor').length === 0) {
       if (typeof header.id === 'undefined' || header.id === '') {
         // to escape characters not allow in css and humanize
-        const id = slugifyWithUTF8(getHeaderContent(header))
+        var id = slugifyWithUTF8(getHeaderContent(header))
+        // to make compatible with GitHub, GitLab, Pandoc and many more
+        if (window.linkifyHeaderStyle !== 'keep-case') {
+          id = id.toLowerCase()
+        }
         header.id = id
       }
       if (!(typeof header.id === 'undefined' || header.id === '')) {

public/js/lib/common/constant.ejs

@@ -6,4 +6,6 @@ window.plantumlServer = '<%- plantumlServer %>'
 
 window.allowedUploadMimeTypes = <%- JSON.stringify(allowedUploadMimeTypes) %>
 
+window.linkifyHeaderStyle = <%- JSON.stringify(linkifyHeaderStyle) %>
+
 window.DROPBOX_APP_KEY = '<%- DROPBOX_APP_KEY %>'