If user unsubsrcribes from a resource that they no longer have
access to they should not be revealed the resource path, but be
redirected to app root instead.

https://gitlab.com/gitlab-org/gitlab-ce/issues/64938

rails_helper.rb's only logic was to require
spec_helper.rb.

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

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 }`

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

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.

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

* Created a force=true param that will continue with the previous
  behaviour of the unsubscribe method
* Created a filter for not-logged users so they see a unsubsribe
  confirmation page
* Added the List-Unsubscribe header on emails so the email client can
  display it on top