Hack to add push priority to push notifications (#7765)

* Remove obsolete comment about ancient temporary code

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

* Implement hack to set push priority

based on whether the tweaks indicate the event might cause
effects.

* Changelog for 7765

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

* Antilint

* Add tests for push priority

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

* Update synapse/push/httppusher.py

Co-authored-by: Brendan Abolivier <babolivier@matrix.org>

* Antilint

* Remove needless invites from tests.

Co-authored-by: Brendan Abolivier <babolivier@matrix.org>