notify about "mentions", add "mention" switch

[r10s]

in groups, Android and iOS meanwhile optionally notify about things that are directly directed to you, even if a group is otherwise muted. we call that "mentions". these are:

• replies to own messages
• reactions to own messages
• webxdc-notifications ¹

there were all kind of additional discussions around these feature, this issue is about bringing desktop to the level of iOS and Android. steps needed:

• adapt the notifiy yes-or-no logic to the new flow (flowchat here). basically, to the existing conditions, add if !isOneToOne AND isDirectedToYou and isMentionsEnabled { do_notify() }
  see eg. here and here for the logic changes on iOS

• add a toggle that to control isMentionsEnabled to the notification settings - for screenshots see here and here

  • the strings are already there as pref_mention_notifications and pref_mention_notifications_explain
  • the toggle should default to ON
  • for storing the option, use set/get_config("ui.mute_mentions_if_muted")

• to make the profile settings easier understandable i would avoid the double negative default of "Mute Notifications = Off" and instead go for a simple "Notifcations = ON" as default, as shown on the screenshots linked above. that would also be more logic with the following "Mentions" toggle, that then can be disabled when notifications are disabled. but that point is not really a blocker, but would also help on unify settings

finally, there is no urgency in this issue :) cc @adbenitez - please amend if i forgot something

Footnotes

1. webxdc-notifications are currently always regarded as being a direct mention, so more important than the noise otherwise happening in groups, see discussions offline and eg. at https://github.com/deltachat/deltachat-android/pull/3456 ↩