Implement MSC3952: intentional mentions (#9983)

Implements the intentional mentions feature of MSC3952 (behind
a labs flag).

If enabled, this will send an org.matrix.msc3952.mentions property
on events that will contain the user IDs and/or whether the room is
being mentioned. These mentions also gets propagated via some
custom behaviour for replies and edits.

src/MatrixClientPeg.ts

@@ -235,6 +235,8 @@ class MatrixClientPegClass implements IMatrixClientPeg {
             SlidingSyncManager.instance.startSpidering(100, 50); // 100 rooms at a time, 50ms apart
         }
 
+        opts.intentionalMentions = SettingsStore.getValue("feature_intentional_mentions");
+
         // Connect the matrix client to the dispatcher and setting handlers
         MatrixActionCreators.start(this.matrixClient);
         MatrixClientBackedSettingsHandler.matrixClient = this.matrixClient;