Skip throttling for requests used to inline CSS in emails

What does this MR do?

Related issue: #22465 (closed)

We are using fphilipe/premailer-rails to inline CSS for emails by making network requests for linked style sheets. Those requests end up in the Unauthenticated request rate limit when enabling Admin Area > Settings > Network > User and IP Rate Limits throttle.

This MR allows skipping requests that start with /assets/mailer- from rate limiting.

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

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