Bug 20581: Modifications to database schema

Bug 20581: Modifications to database schema

- This patch provides an illrequests.status_alias column that is a foreign key to
authorised_values.id. This provides the ability for an ILL request to
have an optional status alias that is defined as an authorised valued.
- A new ILLSTATUS authorised value category is also made available during install.

Signed-off-by: Niamh.Walker-Headon@it-tallaght.ie
Following commit was squashed into this one:

Bug 20581: (follow-up) Modify DB updgrade

As per
https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=20581#c56

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>