Fix browsing file names with percent-encoded @

This adds an encode method into the javascript code so that
users can browser the Godot docs for @globalscope and @GDscript.

The bug was mentioned in:
#1853

This adds the encoding on the frontend, rather than trying to override
filename generation in scrapers. It's possible that this will impact
other documentation sources, but I expect those would also need to have
encoded the @ as %40 in their own docs.

This doesn't use JS' encodeURI or encodeURIComponent because encodeURI
doesn't include @, and encodeURIComponent includes / which we don't
want. If there are other URL-reserved characters
that would be useful to encode, those should be easy enough to add to
the short method added here.

assets/javascripts/models/doc.js

@@ -71,9 +71,14 @@ app.models.Doc = class Doc extends app.Model {
     return this.entry;
   }
 
+  encodePath(path) {
+    return path.replace(/@/g, "%40");
+  }
+
   findEntryByPathAndHash(path, hash) {
     let entry;
-    if (hash && (entry = this.entries.findBy("path", `${path}#${hash}`))) {
+    let encodedPath = this.encodePath(path);
+    if (hash && (entry = this.entries.findBy("path", `${encodedPath}#${hash}`))) {
       return entry;
     } else if (path === "index") {
       return this.toEntry();