Log LDAP blocking/unblocking events to application log

What does this MR do?

Log a message to application.log when a user is blocked/unblocked from LDAP login.

Why was this MR needed?

User blocks can happen due to LDAP misconfigurations or temporary problems with the LDAP backend, so logging these events helps with troubleshooting.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• 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?

• Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6103 (/cc @jacobvosmaer-gitlab, @rymai, @dblessing)