Bug 21846: Add Koha::Tags::Approval(s) and Koha::Tags::Index(es)

Bug 21846: Add Koha::Tags::Approval(s) and Koha::Tags::Index(es)

In order to avoid writing the tests using plain DBIC which would later
need to be replaced by a Koha::Object-based counterpart, I introduce
this stub classes.

Stub tests are added as well.

To test:
- Apply this patch
- Run:
  $ kshell
 k$ prove t/db_dependent/Koha/Tags.t \
          t/db_dependent/Koha/Tags/Approvals.t \
          t/db_dependent/Koha/Tags/Indexes.t
=> SUCCESS: Tests pass
- Sign off :-D

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>