Bug 25629: Fix capitalization in sample notices

Bug 25629: Fix capitalization in sample notices

This fixes the capitalization in sample notices files to
follow coding-guidelines. Only applies to English, translations
remain unchanged.

Additional fixes:
- Overdues slip added the letter code as subject instead of the description
- Opac changed to OPAC for self-registration verification emails

German (de-DE) file was not changed, as there is a pending translation
bug that will take care of these (bug 25129).

To test:
- Verify changes by running the web installer for the changed
  files and verify sample notices install without error
- Verify subjects and notice descriptions are correct
- Run tests in t/db_dependent/Letters*

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