Fix small bug with problem attempt icon

[int-y1]

This ensures that 100/100 WA shows up as a problem attempt.

*_attempted_ids are used in these 3 places:

• /problems/ to display orange icon in table column
• /problems/?order=solved for solve order
• /problem/aplusb to display orange icon in title

In all of these, a set works well enough, and a dict is unnecessary.

[codecov-commenter]

Codecov Report

Base: 46.67% // Head: 46.67% // No change to project coverage 👍

Coverage data is based on head (63acdc4) compared to base (5f2e28e).
Patch coverage: 33.33% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2104   +/-   ##
=======================================
  Coverage   46.67%   46.67%           
=======================================
  Files         240      240           
  Lines       13263    13263           
=======================================
  Hits         6190     6190           
  Misses       7073     7073           

Impacted Files	Coverage Δ
judge/utils/problems.py	26.86% <33.33%> (ø)

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