Bug 27395: Add a warning to PatronSelfRegistrationDefaultCategory

Bug 27395: Add a warning to PatronSelfRegistrationDefaultCategory

This patch adds a warning to the PatronSelfRegistrationDefaultCategory system
preference to not use a regular patron category for self registration.

If a regular patron category code is used and the cleanup_database cronjob is setup
to delete unverified and unfinished OPAC self registrations, it permanently and
and unrecoverably deletes all patrons that have registered more than
PatronSelfRegistrationExpireTemporaryAccountsDelay days ago.

It also removes unnecessary apostrophes at the end of two self registration
and modification system preference descriptions.

Test plan:
1. Apply the patch.
2. Check that the warning message for the PatronSelfRegistrationDefaultCategory
   system preference is clear and makes sense.
3. Sign off!

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>