Improve wallabag:// to prefill authentication

@AndyScherzinger added wallabag:// in #300.

An improvement could be to send more information to the android application: client id, client secret, server URL (thank you @bourvill for the idea).

Start by reading the wallabag:// implementation introduced in #300 and trace how the Android application handles the link. Determine how the client ID, client secret, and server URL should be passed for authentication, then verify that opening a link pre-fills those values without requiring manual entry.

Written by the indexing model from the issue text.