Bug 26734: Rename activethemes with availablethemes

Bug 26734: Rename activethemes with availablethemes

There was an existing call to $self->activethemes that behave as setter:
   85     $self->activethemes($activethemes);

I am not sure what are the side-effects but it does not look good.

It caused warning in the logs:

Use of uninitialized value $interface in string eq at /kohadevbox/koha/C4/Templates.pm line 279.
Use of uninitialized value $interface in string eq at /kohadevbox/koha/C4/Templates.pm line 280.
Use of uninitialized value $interface in string eq at /kohadevbox/koha/C4/Templates.pm line 282.
Use of uninitialized value $lang in concatenation (.) or string at /kohadevbox/koha/C4/Templates.pm line 296.
Use of uninitialized value $lang in concatenation (.) or string at /kohadevbox/koha/C4/Templates.pm line 296.
Use of uninitialized value $lang in concatenation (.) or string at /kohadevbox/koha/C4/Templates.pm line 296

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>