Bug 7046: Implemented subscription renewal dropdown sub length element

Bug 7046: Implemented subscription renewal dropdown sub length element

To make this work I moved the _get_sub_length function from
subscription-add.pl to C4/Serials.pm so that the subscription-renew.pl
script could also call it to store the sublength for the appropriate
field of the subscriptions database table.

Test plan:
1. Create a subscription and notice that there is a dropdown box for sub
   length containing the values: issues, weeks, months
2. Renew the subscription and notice that there are 3 input text boxes:
   'number of num', 'number of weeks' and 'number of months'
3. Input a 'Number of weeks' value of 2
4. Query the subscription database table and notice that the value of 2
   has been stored in the weeklength field for the subscription record you
   just renewed
5. Apply the patch
6. Renew the subscription and notice that there is now a sublength
   dropdown box containing issues, weeks and months
7. Set the month value to 3
8. Query the database and notice that 3 was stored in the monthlength
   field for the subscription record
9. Create a new subscription and select the sub length values of issues
   and 3
10. Query the database and notice that the numberlength field for the
   subscription you just created is set to 3 showing that the sublength
   dropbox is still working for creating a new subscription

Sponsored-By: Catalyst IT
Signed-off-by: Dilan Johnpullé <dilan@calyx.net.au>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>