Bug 12296: make search box customisable with OpacCustomSearch

Bug 12296: make search box customisable with OpacCustomSearch

This allows the search box to be replaced by some custom HTML, useful
when you're needing to integrate with some other search system, and
don't want to maintain a template change across upgrades.

Test plan:
* Install patch
* Look at the OPAC, see that nothing has changed.
* Change the OpacCustomSearch syspref to something like <h1>Zuul</h1>
* Look at the OPAC again, you can no longer search, there is only Zuul.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Works as advertised. Seems like it would be really easy to screw up your
OPAC with this feature, but since we already offer other easy ways to
screw up your OPAC I guess this fits in.

New patch changes: removed the bootstrap code, changed the entry in
syspref.sql

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works as described, passes QA script and tests.
The feature has some limitations, described on the bug.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>