Add restrictions for email addresses

What does this MR do?

Part of https://gitlab.com/gitlab-org/growth/team-tasks/issues/62

Adds another option to restrict email addresses by adding a regex. The regex gets applied to the whole email address. This gives admins a more precise option to forbid certain characters for email addresses on sign ups.

Feature flag: email_restrictions

Follow-ups

• Adjust design like in https://gitlab.com/gitlab-org/growth/team-tasks/issues/62#note_287135174
• Add it to API
• Add documentation
• Remove feature flag

Screenshots

It is in its own section

We validate the regular expression

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