Add option: Auto-retrieve MMS
Created by: SlavikCA
Currently, MMS are always auto-retrieved.
However, in light of "Stagefright" vulnerability: https://nakedsecurity.sophos.com/2015/07/28/the-stagefright-hole-in-android-what-you-need-to-know/
Need to have option to turn off Auto-retrieval of MMS.
Related to this upstream issue: https://github.com/WhisperSystems/TextSecure/issues/3817 However, @moxie0 comment seems to be not correct: as per vulnerability, it doesn't require user to click / open anything...