Change a few specs to run with the new sidebar

What does this MR do and why?

This is all part of the bigger effort in #420119 (closed) (and a little bit of the related #420121 (closed)) in order to be able to remove the super_sidebar_logged_out feature flag. See also: #419936 (closed)

Run Marketing Header spec with super sidebar

This switches the Marketing header specs from the old nav to the new nav.

Run dashboards shortcuts spec with the new nav

Apparently they are passing and no changes are needed

User explores projects spec fixed with new sidebar

We also move the shared example into the spec file, as it was only used here and I don't think it will be used as part of other specs.

Fix two user profile related specs with new sidebar

The new sidebar adds more dropdowns to the page and therefore some CSS selectors had to be made more specific on which dropdown to select.

Furthermore the new sidebar doesn't render a 0 pill if a user doesn't have a follower or is following noone. The tests have been adjusted accordingly.

Screenshots or screen recordings

N/A

How to set up and validate locally

Execute the changed specs locally.

MR acceptance checklist

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