fix: use lowercase email for gravatar

[goenning]

Issue: part of #526

[codecov-io]

Codecov Report

Merging #527 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #527   +/-   ##
=======================================
  Coverage   74.44%   74.44%           
=======================================
  Files          96       96           
  Lines        6365     6365           
=======================================
  Hits         4738     4738           
  Misses       1286     1286           
  Partials      341      341

Impacted Files	Coverage Δ
app/handlers/images.go	60% <100%> (ø)	⬆️

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