Add preference to disable outgoing MMS

Add a preference for completely disabling outgoing MMS.
Separate control for SMS and MMS asking before sending.
Set "ask before sending" as default.
Rename "Allow outgoing SMS/MMS" to make clear that this option also effects MMS.

This commit should protect new users from accidentally sending expensive MMS.

Should close #769

![outgoingsmsmms](https://cloud.githubusercontent.com/assets/6969594/2713595/4c1dac06-c4e9-11e3-8203-a0eb51c15d7e.png)

 I also thought about introducing a separate activity for MMS like the one for outgoing SMS but I think it is better to have those options at the same place.
Maybe it would be good to rename all methods that effect both SMS and MMS to something like "isSmsMmsFallbackEnabled" but I did not want to make too many changes on the existing code.