Figure out how to group notifications

[uscki-gogs-to-github-migrator]

_{This issue was originally created by @AJdeMooij on Saturday 08 September 2018 at 16:40:27 and later updated on Saturday 07 March 2020 at 18:29:51}

Notifications relating to the following should be grouped, if they are about the same item:

• agenda
• meetings
• forum stuff

Keys have been added to NotificationReceiver, but grouping itself is quite difficult, because you need to build a group summary notification first. We can't keep track of the open notifications (yet), so that might be something that has to be solved first.

[uscki-gogs-to-github-migrator]

_{This comment was originally placed by @AJdeMooij on Saturday 07 March 2020 at 18:29:51}

Zie discussie in #105