Prevent duplicate solve submissions for the same scramble

[coder13]

A competitor should not be able to time or submit the same scramble more than once unless explicitly editing their existing result.

Feedback source:

• Disallowing a solver to time the same scramble twice.
• Not allowing a solver to get access to a scramble they previously did on the timing area.
• Bug report: editing a past solve after a new scramble exists can hide the current scramble, as if the edit counted as the current solve.

Acceptance ideas:

• Treat each user/scramble pair as having at most one active result.
• Editing an old result must not mutate current scramble availability.
• Current scramble visibility should depend on the user's current attempt state, not edits to previous attempts.
• Add server-side validation where possible.