[Port dspace-7_x] Add route guard to prevent access to register page for authenticated users

[FrancescoMolinaro]

References

Backporting of #4764 to dspace-7_x

Description

Implemented route guard to prevent logged user from accessing register page.
The user will be redirected on page 404 if already logged in.

Instructions for Reviewers

To test this feature you can try and log in while navigating the register page.
The logged user will be redirected to the "Not found" page.

List of changes in this PR:

Implemented new route guard to check if the user is authenticated.
Adapted route config.

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[tdonohue]

Closing and reopening to trigger tests to re-run. Somehow they got "stuck" in this PR and never ran.

[tdonohue]

👍 Thanks @FrancescoMolinaro ! This backport looks good. Code looks the same and I manually tested it on a 7.x instance and it's working well. I'm waiting on the automated tests to finish re-running (they didn't trigger previously) and then I'll merge it.