auto-detect and set target_locale for langpacks

[eviljeff]

Follow-up from #7908 - we detect the target_locale for dictionaries but not langpacks. We should do something similar for langpacks

[ioanarusiczki]

@eviljeff
I submitted lang packs with dev hub and the new API (PUT or POST) and checked that at creation of a new lang pack the target_locale is present in admin -> this is happening now
The field from admin can be edited by the admin.

I tried to make a new version upload for an older lang pack -> the field remains empty -> it is populated with the locale from the langpack_id (inside manifest.json) only when a new locale is created.
Also, when I change the langpack_id and submit a new version -> nothing changes (the target locale from create is displayed)

I think this should be ok, but if not , let me know.

[eviljeff]

Also, when I change the langpack_id and submit a new version -> nothing changes (the target locale from create is displayed)

We should probably error for that, but langpacks are only submitable by releng anyway, so there's no much risk of accidents.