Bug 27944: Add "requested" stage in article request process

Bug 27944: Add "requested" stage in article request process

This patch adds the stage "requested" in article request process, which
is previous to pending stage.

To test:
1. apply this patch
2. updatedatabase
3. enable ArticleRequests syspref
4. from staff inteface and from opac search for a record and place an
   article request
5. koha-mysql kohadev
6. query: select subject, content, letter_code from message_queue;
CHECK => There is a message for each article request with code
AR_REQUESTED
      => In opac-user.pl, in "Article requests" tab you should see a row
in the table with "Requested" status
5. in staff go to Circulation -> Article Requests
SUCCESS => You should see 3 tabs, one for Requested stage (with two
requests), one for Pending stage and one for Processing stage.
6. play with actions buttons
CHECK => you should see a new action called "Set request as pending"
SUCCESS => All action buttons behave as expected, and tab counts updates
correctly.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>