Allow customization in blocked message

Description

Would like to be able to set a custom message to displayed to a user when a blocked user attempts to login.

"Your account has been blocked. Please contact your GitLab administrator if you think this is an error."

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/models/user.rb#L219

Internal Zendesk: https://gitlab.zendesk.com/agent/tickets/88993