Add path in session cookie when namespace is provided in path

What does this MR do?

Related to: GitLab Pages without DNS wildcard - MVC (gitlab#17584 - closed)
More context: gitlab#211677 (comment 1646965167)

Add path in session cookie when namespace is provided in path. This will prevent cookie sharing when the namespace is provided in the path.
We are setting the path variable at the cookie level so that the browser only sends the specific cookie assigned to that namespace.

Cookie path documentation: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#pathpath-value

This will work in conjunction with: Add namespace_in_path parameter in GitLab.rb fo... (omnibus-gitlab!7250 - merged)

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