Add customisation support for notifications

#### Is your feature request related to a problem? Please describe.
It's always annoying when my phone is forever getting notifs from Element. I want the notifs, but I also want to be able to edit which notifs I get.

#### Describe the solution you'd like.
I'd like to be able to choose between getting all notifs, being notified when mentioned, or having no notifs at all, and toggling notifs per-platform (I want all notifs on my desktop but not my phone).

#### Describe alternatives you've considered.
My current solution is just disabling notifs and checking in every so often.