Correct WEBBIN_ROW_NAMES for bottom bin shown by TaxBrain

[martinholmer]

This pull request resolves PolicyBrain issue 214 by changing the first name in the WEBBIN_ROW_NAMES list from <$0K to $0-10K. And because the dropping of the bin containing filing units with negative income is something done only for TaxBrain, the WEBBIN_ROW_NAMES list has been moved from taxcalc/utils.py to taxcalc/tbi/tbi.py. This move means that TaxBrain will have to import the corrected WEBBIN_ROW_NAMES list from tbi not from utils.

@hdoupe @Amy-Xu

[codecov-io]

Codecov Report

Merging #1889 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1889   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          37      37           
  Lines        3301    3301           
======================================
  Hits         3301    3301

Impacted Files	Coverage Δ
taxcalc/utils.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 044bb80...175c067. Read the comment docs.