Bug 24698: Add plugin and template files for unimarc authority leader

Bug 24698: Add plugin and template files for unimarc authority leader

In Unimarc installations when adding/editing an authority record there is no
Tag Editor for the leader field ( 000 ).

This patch fixes that.

Test plan:

1) Try to catalogue a new authoritry and check if on field 000 you can find
the Tag Editor.
2) Apply the patch.
3) In the authorities framework view the subfields of field 000 and select for
plugin option the value unimarc_leader_authorities.pl.
4) Restart memcached / plack if needed.
5) Repeat step 1 and notice if the Tag Editor is visible and click it to open
new window in order to manage the field contents and press OK.
6) Check the value of the generated leader.

Signed-off-by: Esther <estherm@sodapsringsid.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>