Add methods to return the messages attached to a patron.
It will add the capability to access them from notice templates.
Test plan:
Define some messages for a given patron
Go to the circulation page of the patron and confirm that they are still
displayed
Test the notice templates:
Add to HOLD_SLIP the following content
"""
[% SET messages = borrower.messages %]
[% IF messages.count %]
Messages:
<ul>
[% FOR m IN messages.search( message_type => 'L' ) %]
<li>[% m.message %]</li>
[% END %]
</ul>
[% END %]
"""