Draft: account-details: Allow to configure matrix push notifications

This will become more relevant once !1355 e matures. Marking as draft as the UI will likely change and we want we also need to land https://source.puri.sm/Librem5/libcmatrix/-/merge_requests/56 first.

Missing bits

• Delete pusher when disabling push notifications
• See TODOs in the source

/cc @suraj_sloth This should get you pretty close once you have the DBus interface in place.