deduplicate message notifications

[haraldschilly]

I frequently get mentioned N>1 times for the same message, just because it was edited in the chat. This should be deduplicated. So, at least when the data comes in, only the newest mention for the same message (project_id, path, timestamp) should be used. The overall counter should only add 1 for one message and they shouldn't show up in the notifications multiple times.

A tricky aspect might be the read/save/unread state.