Add page for scanning and sharing QR codes

Currently, the page can only add contacts by scanning QR codes which contain XMPP URIs with their JIDs.

The chat page is opened when the scanned contact is already in the roster. A passive notification is shown when the URI is invalid.

It is intended to be extended by scanning JIDs of group chats and fingerprints of OMEMO keys as soon as those features are implemented.

This is part of #290.

This should be merged after !650 (merged).