Bug 18776: Translatability: Get rid of exposed tt directives in opac-advsearch.tt

Bug 18776: Translatability: Get rid of exposed tt directives in opac-advsearch.tt

The file opac-advsearch.tt exposes template directives to translation where translators should not be confronted with.
Example in po file:
"[%% IF ( ( OpacAdvSearchOptions and OpacAdvSearchOptions.grep('itemtype')."
"size > 0 and not expanded_options ) or ( OpacAdvSearchMoreOptions and "
"OpacAdvSearchMoreOptions.grep('itemtype').size > 0 and expanded_options ) ) "
"%%] "

To test:
- Apply patch
- Verify that advanced search in OPAC behaves as before
- Create a new translation for a 'language' aa-AA (perl translate create aa-AA)
- Verify that template directives ar no longer exposed in aa-AA-opac-bootstrap.po

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit ae684fc9491102ba0b560ca6f414a325e763b31a)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>