Combine and limit delete-notification pushes

[nickvergessen]

Currently all mobile clients always receive all delete-pushes. iOS ignores silent background notifications if there are too many, so we plan to limit and combine them:

• Only send talk to talk apps and all others to the files app
• Combine the multiple deletes into 1 push, so instead of:

   		[
   			'nid' => 13,
   			'delete' => true,
   		]

   		[
   			'nid' => 42,
   			'delete' => true,
   		]

   		[
   			'nid' => 23,
   			'delete' => true,
   		]

the Talk iOS app receives:

   		[
   			'nids' => [13, 42, 23],
   			'delete-multiple' => true,
   		]

[blizzz]

moving to 25

[SystemKeeper]

Fixed by #1429 and included in 26.0.0 ?