feat: add notifications

[luka-nextcloud]

Resolves: #1460

Demo:
demo.webm

[blizzz]

Thanks @luka-nextcloud! I did only little smoke testing, but that was already working. I left some comments around here in the code, the ones marked with 💡 are not blocking, but for consideration.

What I also did not check yet, question back for you, do you think this scales with a bigger number of users, or users in a group?

My impression is that in some places there is code that is quite similar, an almost-duplication. There might be potential to reduce this, but do not consider this critical.

[blizzz]

P.S.: what I forgot to missed to add: 💄 better to not include the row ID in the text of the notification, it does not really help people and looks technical. Just link row, there is a direct link already anyway (at least for the creates and updates).

[enjeck]

not sure if should be done in another PR, but could we have notifications for asynchronous import? Right now, users never know exactly when import is done and it would be great

[enjeck]

some comments

[luka-nextcloud]

not sure if should be done in another PR, but could we have notifications for asynchronous import? Right now, users never know exactly when import is done and it would be great

I think the asynchronous import notification should be another PR since this PR is already big enough.
I've created a followup task here - #2764