Add mentions as links to rte (#10463)

* adds autocomplete behaviour for mentions in rich text editor
* allows adding mentions as links
* adds tests

res/css/views/rooms/wysiwyg_composer/components/_Editor.pcss

@@ -100,6 +100,11 @@ limitations under the License.
                 padding: unset;
             }
         }
+
+        /* this selector represents what will become a pill */
+        a[data-mention-type] {
+            cursor: text;
+        }
     }
 
     .mx_WysiwygComposer_Editor_content_placeholder::before {