Bug 2832 - Make alphabet localizable

Bug 2832 - Make alphabet localizable

Adds the "alphabet" syspref, which can hold a space separated list
of charachters, which can be expanded into browse links when needed.

To test:
- Make sure to run updatedatabase
- Check that the "alphabet" syspref was added under "I18N/L10N preferences"
- Check that browse links are displayed Home › Patrons
- Add some weird charachters like ÆØÅ to "alphabet" and check that the
  browse links are still displayed ok

Signed-off-by: Mirko Tietgen <mirko@abunchofthings.net>
Passed-QA-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>