Fix validation bug on missions update #338

[Creiwry]

Issue:
The enrollments validator was raising an AvailabilityPlaceValidator error when enrollments were updated. Only this issue from the ticket is being addressed here due to priority.

Implemented changes:
The AvailabilityPlaceValidator scope has been changed to on: create, as that is the only time it is helpful to check if the max_member_count condition has been reached.

[Mate2xo]

@Creiwry To be sure we get the intended result, please write a test to simulate what happens on a full slot when another member tries to join in

[Mate2xo]

[...] is created on a full mission
might be better ?

[Mate2xo]

Instead of creating a method helper that is used only once in that file (meaning, it only moves the logic elsewhere), I think this setup can be done directly under this context. You could either create a new trait in the mission factory, or directly set the setup here in a let. Here, the let directly illustrates what is the actual context in terms of code

Suggested change

	it 'raises an error' do
	mission = create :mission, max_member_count: 4
	enroll_n_members_on_mission(mission, 4)
	extra_enrollment = Enrollment.new(mission: mission)
	expect(extra_enrollment).to_not be_valid
	end
	let(:full_mission) do
	create :mission, max_member_count: 4 do |mission|
	create_list :enrollment,
	4,
	mission: mission,
	start_time: mission.start_time,
	end_time: mission.start_date + 3.hours
	end
	end
	end
	it 'is invalid' do
	extra_enrollment = Enrollment.new(mission: full_mission)
	expect(extra_enrollment).to_not be_valid
	end

Also, it's better to be specific if we can on what error is raised. Here, it's an invalidation