GitLab Pages without DNS wildcard - Pages handler implementation

What does this MR do?

GitLab Pages without DNS wildcard - Pages handler implementation

Related: #1111 (closed), #1109 (closed)

Implementation (When namespace-in-path is enabled):

1. We have implemented a handler namespaceinpath/middleware.go which rewrites namespace in path to namespace in host.
2. We have implemented a custom response writer namespaceinpath/responsewriter.go which rewrites location header back to namespace in path before sending it to user.
3. I have added unit tests for individual files.
4. I am adding acceptance tests for acceptance/serving_test.go.
5. I am planning to add remaining acceptance tests such as auth, redirect etc in new MR as this MR is getting bigger.

Note: Omnibus and charts will require changes to support non-wildcard urls which will be addressed in separate issue.

Changelog: added

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• 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