Bug 14022: Does not reset the subscription length on editing a subscription

Bug 14022: Does not reset the subscription length on editing a subscription

The subscription length is always set to the first/default value
('issues') when editing a subscription.
Even if you selected another value on creating/editing it, the form will
show you the first option.

Test plan:
1/ Add/Edit a subscription, select a subscription length different of
the first value (issues)
2/ Edit the subscription and confirm the value is kept

Bonus: issues, week and months become translatable.

Works as expected.
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>