auth: Allow '.' in NormalizeUsername

Created by: tsenart

Fixes #4674

Created by: codecov[bot]

Codecov Report

Merging #4690 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4690      +/-   ##
==========================================
+ Coverage   47.83%   47.83%   +<.01%     
==========================================
  Files         725      725              
  Lines       44235    44235              
  Branches     1758     1758              
==========================================
+ Hits        21158    21160       +2     
+ Misses      21084    21083       -1     
+ Partials     1993     1992       -1

Impacted Files	Coverage Δ
cmd/frontend/auth/auth.go	29.62% <100%> (ø)
...d/frontend/internal/authz/bitbucketserver/store.go	79.88% <0%> (+1.18%)