Ensure all javascripts use GitLab install directory as the cookie path.

What does this MR do?

Fixes #20435 (closed) and standardizes all JavaScript-set cookies to use the same root url.

Are there points in the code the reviewer needs to double check?

Nothing I can think of. This MR is pretty straight forward.

Why was this MR needed?

Cookie paths were inconsistent across front-end scripts. Some would default to '/' and others had no set path and would end up tied to whatever page they were set on. Also as @connorshea noted, we cannot assume GitLab is installed at '/'.

Before:

After:

What are the relevant issue numbers?

#20435 (closed)

Screenshots (if relevant)

see above

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together