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)

db/migrate/20170626202753_update_authorized_keys_file.rb

@@ -17,6 +17,8 @@ class ApplicationSetting < ActiveRecord::Base
 
   def up
     if authorized_keys_file_in_use_and_stale?
+      say 'The authorized_keys file is in use, and may be stale. Now bringing it up-to-date in the background...'
+
       # Update nil authorized_keys_enabled to true to ensure that Gitlab::Shell
       # key methods work properly for workers running 9.3.0 during the
       # migration. If the setting remained nil, the workers would not edit the
@@ -24,6 +26,8 @@ def up
       update_nil_setting_to_true
 
       update_authorized_keys_file_since(DATETIME_9_3_0_RELEASED)
+    else
+      say 'The authorized_keys file does not need to be updated. Skipping...'
     end
   end
 
@@ -62,7 +66,10 @@ def ran_broken_migration?
     # If the column is already fixed, then the migration wasn't run before now.
     default_value = Gitlab::Database.postgresql? ? 'true' : '1'
 
-    !column_exists?(:application_settings, :authorized_keys_enabled, :boolean, default: default_value, null: false)
+    column_has_no_default = !column_exists?(:application_settings, :authorized_keys_enabled, :boolean, default: default_value, null: false)
+    say "This GitLab installation was #{ 'never ' unless column_has_no_default }upgraded to exactly version 9.3.0."
+
+    column_has_no_default
   end
 
   def false_negative_warning