Outgoing MMS preference

Alternative implementation for #1374
Closes #769

<ul>
<li> Add outgoing MMS preference similar to the one for SMS </li>
<li> Change default preference to "ask before sending SMS/MMS" </li>
<li> Add "do not ask again" Checkbox to Fallback confirmation dialog </li>
<li> Bugfix: You were able to send SMS to non Textsecure users if you enable SMS Fallback but disable SMS to non Textsecure users </li>
</ul>