Allow adding 1:1 Talk conversations when adding an existing conversation

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

When I book a meeting with someone on Nextcloud, I just find it convenient to add the 1:1 talk conversation link in. This works just fine if you find and copy it in - the fact that you have a meeting will even show up in that 1:1 talk conversation in Talk.
Buuuut you can't select 1:1 conversations from the list of conversations in the Calendar, so you manually have to go to talk, search for your colleague, copy the link to the conversation and paste it in.

Describe the solution you'd like

1:1 conversations should be there too. Of course this only makes sense for a 1:1 meeting with another Nextcloud user and there's no need to make changes to the description etc!

Describe alternatives you've considered

Keep doing it manually?

Additional context

No response

No file or test is named in the issue. Start with the Calendar UI flow for adding an existing conversation and trace how its conversation list is populated. Done means eligible 1:1 Talk conversations appear for a matching meeting while the existing description behavior remains unchanged.