Convert alias links in room header topics to local permalinks

Fixes https://github.com/vector-im/riot-web/issues/12605

src/linkify-matrix.js

@@ -16,7 +16,7 @@ limitations under the License.
 */
 
 import {baseUrl} from "./utils/permalinks/SpecPermalinkConstructor";
-import {tryTransformPermalinkToLocalHref} from "./utils/permalinks/Permalinks";
+import {tryTransformEntityToPermalink, tryTransformPermalinkToLocalHref} from "./utils/permalinks/Permalinks";
 
 function matrixLinkify(linkify) {
     // Text tokens
@@ -221,7 +221,7 @@ matrixLinkify.options = {
             case 'userid':
             case 'groupid':
             default: {
-                return tryTransformPermalinkToLocalHref(href);
+                return tryTransformEntityToPermalink(href);
             }
         }
     },

src/utils/permalinks/Permalinks.js

@@ -290,6 +290,25 @@ export function isPermalinkHost(host: string): boolean {
     return getPermalinkConstructor().isPermalinkHost(host);
 }
 
+/**
+ * Transforms an entity (permalink, room alias, user ID, etc) into a local URL
+ * if possible. If the given entity is not found to be valid enough to be converted
+ * then a null value will be returned.
+ * @param {string} entity The entity to transform.
+ * @returns {string|null} The transformed permalink or null if unable.
+ */
+export function tryTransformEntityToPermalink(entity: string): string {
+    if (!entity) return null;
+
+    // Check to see if it is a bare entity for starters
+    if (entity[0] === '#' || entity[0] === '!') return makeRoomPermalink(entity);
+    if (entity[0] === '@') return makeUserPermalink(entity);
+    if (entity[0] === '+') return makeGroupPermalink(entity);
+
+    // Then try and merge it into a permalink
+    return tryTransformPermalinkToLocalHref(entity);
+}
+
 /**
  * Transforms a permalink (or possible permalink) into a local URL if possible. If
  * the given permalink is found to not be a permalink, it'll be returned unaltered.