Bug 36758: Add librarian object

Bug 36758: Add librarian object

This patch adds a 'librarian' object fetch to
C4::Letters::GetPreparedLetter when a called passes the 'want_librarian'
flag.

This allows for the notice to take full advantage of the patron object
for that librarian rather than requireing old non-TT syntax for this
feature.

Test plan
1) We use the 'librarian' object in the new TICKET_ASSIGNED default
   notice, use the next patch to test that the librarian title is
   correctly substituted into the notice.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>