Implement composer completion user/room pill insertion

This modifies the composer completion such that completing a room or user will insert an IMMUTABLE matrix.to LINK Entity for the range that was replaced. Display names will not have a colon after their name anymore as it seemed strange that we would insert one after a pill.

src/autocomplete/Autocompleter.js

@@ -34,6 +34,12 @@ export type Completion = {
     component: ?Component,
     range: SelectionRange,
     command: ?string,
+    // An entity applied during the replacement (using draftjs@0.8.1 Entity.create)
+    entity: ? {
+        type: string,
+        mutability: string,
+        data: ?Object,
+    },
 };
 
 const PROVIDERS = [