Bug 23064: Use Koha::Subscription in ModSubscription

Bug 23064: Use Koha::Subscription in ModSubscription

We must use Koha::Subscription instead of raw SQL.
It will fix issue with default and integer values.

Test plan:
Edit a subscription and set number of issues = "f"
Save
=> Without this patch there is a SQL error in the log:
  Incorrect integer value: 'f' for column 'numberlength'
=> With this patch the other changes are effective.

Note: We also could change the type attribute of the input to "number",
to have a client-side check

Also, the return value of ModSuggestion is never used, so we are safe
with that.

Signed-off-by: Hayley Mapley <hayleymapley@catalyst.net.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>