Refactor recipient picker to use moodleform's autocomplete element and web services.

[troywilliams]

The current recipient picker uses the deprecated JavaScript YUI library https://docs.moodle.org/dev/YUI and old-school AJAX calls.

Move to moodleform's autocomplete element and web services API for datasource.

https://docs.moodle.org/dev/lib/formslib.php_Form_Definition#autocomplete

Related issues:

• No drop-down list in search field showing when many users in course #82
• The down arrow stops working when having searched for user to start dialogue with #81
• Show username (or email) in search list #80
• mod_dialogue: Unable to select more than one student when creating a new message in a Dialogue #75
• Request: person search to include role assignments/other users #62
• Differentiate student teacher #54