RecaptchaExperimentHelper contains helper methods
to assist in the controller and view layers.

Allows instance owners to toggle the recaptcha requirement
on the user registration page by feature flag.
Allows GitLab Growth team to measure
reCAPTCHA's impact on registrations.

Adds frozen string to the following:

* spec/bin/**/*.rb
* spec/config/**/*.rb
* spec/controllers/**/*.rb

xref https://gitlab.com/gitlab-org/gitlab-ce/issues/59758

So the fake can enjoy it, too. We don't use `prepend`
because that'll require we change `allow_any_instance_of` to
`expect_next_instance_of`, but that's not very easy to do.
We can do that later.

Updates specs to use new rails5 format.

The old format:
`get :show, { some: params }, { some: headers }`

The new format:
`get :show, params: { some: params }, headers: { some: headers }`

When a user checks the `accept` checkbox, we will track that
acceptance as usual. That way they don't need to accept again after
they complete the registration.

When an unauthenticated user visits the `/-/users/terms` page, there
is no button to accept, decline or continue. The 'current-user menu'
is also hidden from the top bar.

Signed-off-by: NRémy Coutable <remy@rymai.me>

If a user presses the 'Register' button too quickly after attempting
to solve the reCAPTCHA, or the reCAPTCHA is not solved at all, the
user would experience a 500 error. Now, the case is properly
handled and the user will be sent back to the registration page
with a clear error message and can try again.

- Look for a `change` in its size rather than asserting against an
  actual size.

- This previously failed because another spec had an email in
  `ActionMailer::Base.deliveries`, which failed this `be_nil` assertion.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Also removes the note from the development/testing.md guide