Ban pointless Unicode characters from problem statements

[int-y1]

• i saw ff in a problem statement
• i saw a problem that said Output ... `−1` if it is not possible.

[codecov-commenter]

Codecov Report

Merging #1960 (80b5bde) into master (8eed5e5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1960   +/-   ##
=======================================
  Coverage   46.46%   46.46%           
=======================================
  Files         236      236           
  Lines       13114    13114           
=======================================
  Hits         6093     6093           
  Misses       7021     7021           

Impacted Files	Coverage Δ
dmoj/settings.py	97.70% <100.00%> (ø)

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[Xyene]

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