Fix incorrect user bias transformation

[mdekstrand]

The root cause of #258 seems to be a bug in the bias transformation, in which item IDs were incorrectly used instead of user IDs to look up user biases.

This closes #258.

[codecov]

Codecov Report

Merging #265 (616c5f9) into main (2e7de3a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #265   +/-   ##
=======================================
  Coverage   91.27%   91.27%           
=======================================
  Files          45       45           
  Lines        3761     3761           
=======================================
  Hits         3433     3433           
  Misses        328      328           

Impacted Files	Coverage Δ
lenskit/algorithms/als.py	98.06% <100.00%> (ø)
lenskit/algorithms/bias.py	100.00% <100.00%> (ø)

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[codeclimate]

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