fix: Let's Encrypt integration with /* redirects

What does this MR do?

Let's Encrypt integration relies on acme challenges being redirected to main GitLab server and served there.

We also allow serving ACME challenges from project content just in case users implemented Let's Encrypt integration manually.

But when user adds /* -> redirect_url to .redirects, it treated as project content and will handles as redirect.

This MR just prevents any acme challenges from being processed by the redirects package.

There are other ways to fix it, see the issue description in #649 (closed), but I think this one is the easiest/doesn't break anything/doesn't slow down LE integration.

Related to gitlab#342781 (closed)

TODO

• I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines

Closes #649 (closed)