Bug 9136: C4::Tags not Plack-compatible

Bug 9136: C4::Tags not Plack-compatible

The three module-scoped variables $ext_dict, @fields, and $select_all
were preventing tagging code from working under Plack. I fixed this
by changing the latter two to compile-time constants, and declared the
first with "our $ext_dict;"

To test (under Plack):
1) Try to create a tag before the patch is applied. Note that you get a
   500 error in the AJAX request.
2) Apply patch.
3) Repeat step (1), noticing that this time the tag is created and there
   is no error.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>