Add type to ConversationModel's contactCollection

Previously, this was `Backbone.Collection<WhatIsThis>`. Now, it's
`Backbone.Collection<ConversationModel>`.

A minor change, but an easy improvement that (luckily) had no
downstream effects.

ts/models/conversations.ts

@@ -110,7 +110,7 @@ export class ConversationModel extends window.Backbone.Model<
     { senderId: string; timer: NodeJS.Timer }
   >;
 
-  contactCollection?: Backbone.Collection<WhatIsThis>;
+  contactCollection?: Backbone.Collection<ConversationModel>;
 
   debouncedUpdateLastMessage?: () => void;