Handle locked accounts gracefully

[davidcornu]

Summary of the problem

When we try to create a session for a locked user

hcb/app/helpers/sessions_helper.rb

Line 55 in 21ec8d1

user_session.save!

we end up hitting a validation error

hcb/app/models/user_session.rb

Lines 115 to 119 in a4b1cf6

	def user_is_unlocked
	if user.locked? && !impersonated?
	errors.add(:user, "Your HCB account has been locked.")
	end
	end

which goes unhandled (https://appsignal.com/hack-club/sites/6596247683eb67648f30f807/exceptions/incidents/2036)

Describe your changes

• Checks whether the user is locked in SessionsHelper#sign_in and throws AccountLockedError
• Adds handling for AccountLockedError in LoginsController#complete
• Adds tests to make sure
  • We correctly redirect users
  • We still allow admins to impersonate locked users

[sampoder]

lgtm! thank you!