This PR adds XMPP transport to send secure messages only.

TODO/Features:

• Add XMPP page into intro screen
• Add new "XMPP address" message type to do not use a centralized directory
• Add new XMPP message type when transport is set to XMPP
• Switch to secure SMS if the recipient is offline
• Allow user to use a custom XMPP server
• Delete XMPP account if the user disables XMPP feature
• Display a warning if the custom XMPP server doesn't support XEP-0198 Stream Management (required to do not lost messages when switching from a network to another or on unreliable connectivities)
• Fix bad encryption with long XMPP messages
• Display an XMPP icon for XMPP messages
• Ask the user to optionally set up XMPP on first boot
• Improve XMPP address sharing mechanism
• Share XMPP address when a secure session is started
• Implement a form to manually set up an XMPP address
• Fix recipient status (sometimes a recipient is seen as offline but it is actually online)
• Add support for XMPP media messages (for now, Silence crashes)
• Write documentation
• Include a list of trusted XMPP servers to allow users to register
• OMEMO?

This PR fixes #370 as a side effect because it starts Silence on boot. For now, the list of "trusted XMPP servers" only contains xmpp.silence.im, which is not designed to be used except for testing. We won't host an XMPP server in the short term.