Archive messages

**Is your feature request related to a problem? Please describe.**
Notifications vary in importance and retention needs. It would be nice to archive/divert a message away before deleting all messages (yes I read your comment and agree about short message lifetime, but some are nice to save).

**Describe the solution you'd like**
Ability to archive the message. Archived messages should not be deleted when clicking "DELETE ALL" messages (that I like to click :smile: ). Similar to how https://github.com/usememos/memos does it ( just one large 'Archived' tab that contains them all).

But of course these archived messages should still be delete-able, either moving them back to "main" messages, or a "DELETE ALL ARCHIVED" or whatever.

**Describe alternatives you've considered**
At least individually or in bulk selection: export feature that downloads a json file or whatever of the messages for retention.

**Additional context**
I am aware of and read:
https://github.com/gotify/server/issues/518
https://github.com/gotify/android/issues/303
https://github.com/gotify/android/issues/260