Refactor spec to cache created plan records

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

This introduces a persistent `default`
and `free` plans into database.

The `default` is used by all on-premise
installations. The `free` is created for
GitLab.com.

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.

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.

Enables frozen string in the following:

* ee/lib/*.rb
* ee/lib/api/**/*.rb
* ee/lib/audit/**/*.rb
* ee/lib/banzai/**/*.rb
* ee/lib/ee/**/*.rb
* ee/lib/elasticsearch/**/*.rb
* ee/lib/generators/**/*.rb

Also enables frozen string in some vestigial
files of the ee/app directory.

Partially addresses gitlab-org/gitlab-ce/#47424.