Resolve "Spec failure: spec/features/users_spec.rb"

What does this MR do?

This solves failures that would happen since we were hard-coding user1 as a user username, but this is a problem now that we use the deletion DB cleaning strategy as sequences are not reset between each examples.

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

I've taken the opportunity to reorganize a bit the user-related feature specs, and moved a block of specs from features to routing specs since they were asserting route redirections!

Why was this MR needed?

To fix a ~"broken master".

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #43495 (closed)