Bug 20581: (follow-up) Do not use PK for AV FK

Bug 20581: (follow-up) Do not use PK for AV FK

As per:
https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=20581#c133

We switch from using authorised_values.id when creating the foreign key
illrequests.status_alias, we now use authorised_values.authorised_value

I have added a migration from using id -> authorised_value, so when
existing users of this bug get this version, their DB migrates nicely
instead of just breaking.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>