insert command completion as command part (instead of plain)

this prevents the command being sent as plain text

this adds a `type` property to completions to decide which
parts should be inserted into the composer, hence deciding how
they will be rendered.

src/autocomplete/RoomProvider.js

@@ -89,6 +89,7 @@ export default class RoomProvider extends AutocompleteProvider {
                 return {
                     completion: displayAlias,
                     completionId: displayAlias,
+                    type: "room",
                     suffix: ' ',
                     href: makeRoomPermalink(displayAlias),
                     component: (