WIP: Acme client

What does this MR do?

Implements first part of adding Let's Encrypt support for pages https://gitlab.com/gitlab-org/gitlab-ce/issues/28996

• Adds settings for Let's Encrypt in application settings:
  • notification_email
  • terms of service agreement acceptance
  • private_key (not visible to user)
  • account_id (just cache for eliminating additional requests)
• Adds acme-client gem and build client from settings

Settings at admin/application_settings/preferences#js-pages-settings:

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/28996

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer