Return API error when inviting email restricted for sign-up

What does this MR do?

Refactors the validation used during sign-up for a new user, so that it is also checked when inviting a new email address through the invitations API that will not be able to sign up with that email address.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/321706#note_610592910

Related to !61425 (merged)

Requires: !56957 (merged)

Screenshots (strongly suggested)

Before:

invitation can be sent and clicked on	displays User validation error when trying to sign up with restricted email from the invitation

After:

admin restriction setting	User validation error returned through invitations API	User validation through signup page (unchanged)
Email restrictions:
Domain denylist:
Allowed domains for sign-up:

Note: The modal error messages for all 3 above are planned to be changed in !61425 (merged) so all 3 will instead display:

for admin users:

The domain for {{email address}} is not allowed for this group. Go to the Admin area, and check the Sign-up restrictions.

and for non-admin users:

The domain for {{email address}} is not allowed for this group. Check with your administrator.

group restriction setting	after (unchanged)
Restrict membership by email domain:

Note: The modal error message is planned to be changed in !61425 (merged) so it will instead display:

The domain for {{email address}} is not allowed for this group. Check the permissions on the group’s Settings > General page.

or as an admin:

The domain for {{email address}} is not allowed for this group. Check the permissions on the group’s Settings > General page, or check the Sign-up restrictions in the Admin area.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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