Cache the number of user SSH keys

What does this MR do?

This caches the number of personal SSH keys (as used by e.g. User#require_ssh_key?) in order to reduce the number of SQL queries necessary in actions such as ProjectsController#show.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/36878 for more information.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered