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/NotifProvider.js

@@ -42,6 +42,7 @@ export default class NotifProvider extends AutocompleteProvider {
             return [{
                 completion: '@room',
                 completionId: '@room',
+                type: "at-room",
                 suffix: ' ',
                 component: (
                     <PillCompletion initialComponent={<RoomAvatar width={24} height={24} room={this.room} />} title="@room" description={_t("Notify the whole room")} />