Add project prefix in session cookie path

What does this MR do?

Add project prefix in session cookie path

Setting path variable at cookie level avoids leaking restricted and private projects/subgroups pages under the same top level group.
I have kept this under feature flag: FF_ENABLE_PROJECT_PREFIX_COOKIE_PATH
Related issue: Tech Eval: Set path variable at cookie level to... (#1088 - closed)

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

Changelog: added

Recording:

Before this fix	After this fix
Before Session Cookie Path.mov	After Session Cookie Path.mov

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