Refactor spec to cache created plan records

Ran: bundle exec rubocop --only RSpec/EmptyLineAfterFinalLet -a ee/

Gitlab_subscriptions table currently doesn't keep history. That makes
blocks data team from measuring some growth KPIs. This MR adds a
history log table

Using the sed script from
https://gitlab.com/gitlab-org/gitlab-ce/issues/59758

It adds an upgrade button to the group billings
page that redirects to the customers portal
upgrade checkout page.

If the user is not logged in at the portal he/she
will be automatically redirected after the login
process.

If the user is at the latest tier, the button
won't be presented.

These are not required because MySQL is not
supported anymore

Makes it Date.today when instantiated and not when the
class was loaded

The GL.com plan is no longer stored with Namespace, now this attribute
is part of the new GitlabSubscription model.