Improvements to user member selection

[FelberMartin]

As mentioned in #541

1. We could display a small info box when creating a new chat, to inform the users that a DM will be created when only one user is selected, otherwise a group chat
2. When adding members to an existing conversation, I can still select users that are already members

[julian-wls]

Note regarding 2.: I realized that when adding members on the web app it is also possible to select users that are already members

[FelberMartin]

Oh, thats bad, maybe we can also create an issue for that