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

Store Let's Encrypt account email in application settings
Also add explicit terms of service consent

This reverts merge request !26823

- Update PO file

This replaces the repository param.
This allows more flexiblity as sometimes we have highlight content
not related to repository. Sometimes we know ahead of time the language
of the content. Lastly language determination seems better fit as a
logic in the Blob class.
`repository` param is only used to determine the language, which seems
to be the responsiblity of Blob.

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.

We remove this feature as it never worked properly