Move specs in volunteers_controller_spec to volunteers_spec

[LeGorge]

What github issue is this PR for, if any?

Part of #4129
Not closing yet as there will be more on the way.

What changed, and why?

The volunteers_controller_spec file was removed and all of its specs moved to volunteers_spec

• Most of it was already being tested in other places, so I only added whatever was missing

  • line 32: already tested between volunteer_policy_spec & application_controller_spec
  • line 42: already tested in volunteers_spec at describe "PATCH /deactivate"

• I took the liberty of improving the volunteers edit action:

  • by adding a condition to an assignment as the same condition gates its usage in the view and there's no need to always query the DB for it.

How will this affect user permissions?

• no effect

How is this tested? (please write tests!) 💖💪

• does not apply

Screenshots please :)

• does not apply

Feedback please? (optional)

• I would like to see the request specs renamed to a format including the name request, for example:
  • volunteers_spec to volunteers_request_spec

That would follow rspec's standard naming convention and avoid a wrong connection to volunteer_spec which is the model one.
I'm not sure if that's already planned, because some files are named that way, but I chose to follow the instructions from here: https://github.com/rubyforgood/casa/blob/main/doc/architecture-decisions/0008-controller-specs.txt