Add database column for governing admin approval after user signup

What does this MR do?

For #257881 (closed)

This MR adds the column that controls the setting:

• if enabled, admins will be required to manually approve any user that signs up on the instance, before they can start accessing GitLab
• if disabled, no admin approval is required (this is the current behaviour)

Up migration

▶ VERBOSE=true rake db:migrate
== 20200929063159 AddRequireAdminApprovalAfterUserSignupToApplicationSettings: migrating
-- add_column(:application_settings, :require_admin_approval_after_user_signup, :boolean, {:default=>false, :null=>false})
   -> 0.0087s
== 20200929063159 AddRequireAdminApprovalAfterUserSignupToApplicationSettings: migrated (0.0087s)

Down migration

▶ VERBOSE=true rake db:rollback STEP=1
== 20200929063159 AddRequireAdminApprovalAfterUserSignupToApplicationSettings: reverting
-- remove_column(:application_settings, :require_admin_approval_after_user_signup, :boolean, {:default=>false, :null=>false})
   -> 0.0046s
== 20200929063159 AddRequireAdminApprovalAfterUserSignupToApplicationSettings: reverted (0.0094s)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team