Bug 27946: Add article request fee to circulation rules

Bug 27946: Add article request fee to circulation rules

This patch adds a new circulation rule: article requests fee.

It can be set per library and category.

To test:
1. Try setting sdifferent numeric values, change, and delete them
=> SUCCESS: Things work as expected
2. Make sure rules are loaded correctly when re-entering the circ rules
   page
=> SUCCESS: All good
3. Verify the data on the DB is stored as it should on each of your
   tests:
   $ koha-mysql kohadev
   > SELECT * FROM circulation_rules \
     WHERE rule_name='article_request_fee';
=> SUCCESS: All good (remember NULL => 'All' in the UI)
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>