Synchronise roster with Contacts app

[LEDfan]

TODO:

• check in PHP if contacts app is enabled
• check if there are new contacts which needs to be pushed
• add contactscontroller
• return just created contacts as Chat contacts
• make the BOSH_SERVICE url a config paramter

[LEDfan]

Current state: when you load the chat app, the roster is fetched and saved in the contacts app. The contacts are listed in the conversation list too.

So when you connect to your facebook account, you friends are listed in the left sidebar, and you can send them messages.

[LEDfan]

This PR depends on owncloud/contacts#706

[LEDfan]

This PR depends on owncloud/core#12369

[LEDfan]

Buddies in the roster are automatically added to the contacts. Contacts which support XMPP have a button to add the contact to the roster.