[design] simplify settings

[stefan-niedermann]

The settings are currently way too complicated for people who just want to sync their stuff from phone to their cloud instance.

• [ ] Merge all "Sync via W-LAN / LTE / UMTS / GSM / GPRS" into one switch (not checkbox) "Sync only via W-LAN", because what is the purpose of a sync app, if it does not sync? Sync should always be performed when on W-LAN and just check optionally if user wants to sync on mobile data, too (does not matter wheter LTE or GPRS is used)
• [ ] Use a switch instead of a checkbox for "Forware SMS on receive"
• [ ] "Minimum Phone Number length" - I really can not imagine a usecase except you do not want to sync automatically sent sms from some technic service. Can we make some kind of slider which chooses between minimum 3, 5 and 8 numbers? I do not think the entries 0 - 3 do really make sense.