Don't highlight mention of unknown user

Follow up from here: https://github.com/nextcloud/spreed/issues/1006

This is what happens if I mention a user who doesn't exists in Talk:

I think instead we shouldn't format the mention and show it as a normal string. Imagine I want to tell someone I'm @schiessle on Twitter, this shouldn't be changed to I'm @UnknownUser at Twitter

Some additional thoughts from @jancborchardt :

Agree with @schiessle here. There’s some other cases additionally:

• You simply have a typo in the username: @schiesle or @blizzzz would be completely made unreadable to @Unknown user, which is worse than showing them with a typo
• The case of LDAP you bring up we can easily catch as the LDAP ID is a format we can parse. For those non-existent LDAP users we can indeed show @Unknown user
• In the case where you do not use LDAP but Nextcloud user management and a user is deleted, then showing @Unknown user will make the context invisible too. With a non-marked-up @ivan you at least know who it was addressed too, even though they aren’t in the system anymore.

Start at the Talk mention-rendering entry point and review the linked issue #1006 for the existing behavior and context. Done means unknown mentions remain readable as ordinary text, while the issue's described LDAP exception is handled as intended.