migrate tests to midje and some kondo cleanup

[gkspranger]

• src/yetibot/core/models/users.clj
  • dropped unused refs
  • prefixed unused bindings with _
• test/yetibot/core/test/models/users.clj
  • migrated all tests to midje
  • added some tests

[codecov[bot]]

Codecov Report

Merging #186 (76f29ff) into master (7b982dc) will increase coverage by 0.24%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   53.63%   53.87%   +0.24%     
==========================================
  Files          94       94              
  Lines        4130     4130              
  Branches      212      208       -4     
==========================================
+ Hits         2215     2225      +10     
+ Misses       1703     1697       -6     
+ Partials      212      208       -4     

Impacted Files	Coverage Δ
src/yetibot/core/models/users.clj	77.41% <75.00%> (+16.12%)	:arrow_up:

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