Improve guest handling in comments
Improve guest handling in comments

1. No contacts menu for guests (can't use it anyway)
2. No contacts menu on guests (no data available)
3. Show "You" as author name above "new" box
4. Do not try to load avatars of guests (results in 404)

Signed-off-by: Joas Schilling <ab43a7c9cb5b2380afc4ddf8b3e2583169b39a02@schilljs.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed