Make it possible to mention people on embedded text editors

[blizzz]

Is your feature request related to a problem? Please describe.

At the moment, to be able to mention people needs a test session as well as a document.

When being in an embedded environment however, for instance in a Deck board or in a Tables call, we have neither of them.

Autocompletion and notification may depend on the scope of the embedded environment (e.g. shared state) so a callback possibility seems to be necessary.

[juliusknorr]

@max-nextcloud Just to not forget about it, as raised via chat, we dropped the placeholder in #6855, but I'm unsure if there was discussion with designers about that