Fix the redirect to custom home page URL and move it to RootController

What does this MR do?

It fixes the redirection to a custom Home Page URL.

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

The redirection was being done in the overriden ApplicationController#authenticate_user! method which was unexpected.

Then !8864 (merged) broke the redirection because it skipped the authenticate_user! before filter (which shouldn't have had this unexpected consequence).

I've moved the logic to RootController#index since it's only supposed to happen there. I hope we have enough test coverage and that I'm not breaking something else... We definitely missed some basic controller specs (which I've added now) for this feature.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #28609 (closed)