Mentions: Add a warning on servers where MSC 4028 is not supported

[Velin92]

We check through the API I exposed matrix-org/matrix-rust-sdk#2840 if the homeserver supports MSC 4028 and if not, an alert saying something like "be aware that your homeserver does not support the Mentions only options for ecrypted rooms which may result in the loss of all pushes if enabled" (final copy TBD) is shown.

This should be shown in Advanced Settings -> Notifications -> Group or Direct, as soon as the view is loaded
This should also be shown in Room Details -> Notifications -> Custom Settings on, but only and only if the room is encrypted otherwise we can avoid that

We still allow the user to do any change, we just inform them that it might affect encrypted rooms.

[Velin92]

Kamino cloned this issue to vector-im/element-x-android