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/CommunityProvider.js

@@ -84,6 +84,7 @@ export default class CommunityProvider extends AutocompleteProvider {
             ]).map(({avatarUrl, groupId, name}) => ({
                 completion: groupId,
                 suffix: ' ',
+                type: "community",
                 href: makeGroupPermalink(groupId),
                 component: (
                     <PillCompletion initialComponent={