Correctly return message thread id for a pair and current user

[tibetsprague]

Fixes: Hylozoic/hylo-evo#748 and #585

[codecov]

Codecov Report

Merging #738 (b0034e9) into dev (43c1bf5) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##              dev     #738   +/-   ##
=======================================
  Coverage   58.27%   58.27%           
=======================================
  Files         203      203           
  Lines        5560     5560           
=======================================
  Hits         3240     3240           
  Misses       2320     2320           

Impacted Files	Coverage Δ
api/models/User.js	60.90% <0.00%> (ø)

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[lorenjohnson]

Yay! Was hoping it was something simple like that but didn't get to that on my first pass way back whenever.