Default `authorized_keys_enabled` setting to true

What does this MR do?

Sets the column default for the Application Setting authorized_keys_enabled to true and rebuilds the authorized_keys file.

Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ee/issues/2738

EE installations that were upgraded to 9.3 stopped writing to the authorized_keys file by default. The setting authorized_keys_enabled was nil. So additions and deletions of SSH keys had no effect on SSH operations.

When I added the setting, I mistakenly thought settings were merged with ApplicationSetting.defaults. However, if there exists an ApplicationSetting record, defaults is never used.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #2738 (closed)