Updating multiple application settings panels through
a single action causes the incorrect action to be shown
when there are errors.  Instead, make each panel action
handle both updating and display.

Part of adding Let's Encrypt certificates for pages domains

Add acme-client gem

Client is being initialized by private key stored in secrets.yml
Let's Encrypt account is being created lazily.
If it's already created, Acme::Client just gets account_kid by
calling new_account method

Make Let's Encrypt client an instance
Wrap order and challenge classes

Instance level clusters were already mostly supported, this change adds
admin area controllers for cluster CRUD

Was introduced in the time that GitLab still used NFS, which is not
required anymore in most cases. By removing this, the API it calls will
return empty responses. This interface has to be removed in the next
major release, expected to be 12.0.

This icommit adds several changes related to the same topic
- resetting a Runner registration token:

1. On Project settings page it adds a button for resetting the
   registration token and it removes the Runner token field
   that was confusing all GitLab users.

2. On Group settings page it adds the same button for resetting
   the registration token.

3. On Admin Runners settings page it moves the button to the same
   place as in Project and Group settings and it changes slightly
   the page layout to make it more similar to Group and Project
   setting pages.

4. It refactorizes a little the partial that prints runner
   registration description. Thanks to this Project, Group
   and Admin settings of the Runner are re-using the same
   code to generate the button.

5. Updates the translations of changed text.