fix: discord avatar override

[piksel]

Fixes Discord avatar override not being populated in the webhook payload. Also, this fixes the documentation name of the prop (and adds an alias for avatarurl which was listed in the docs previosuly).

[codecov]

Codecov Report

Merging #172 (6bf798b) into main (d8371b8) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
+ Coverage   63.08%   63.10%   +0.01%     
==========================================
  Files          79       79              
  Lines        2487     2488       +1     
==========================================
+ Hits         1569     1570       +1     
  Misses        780      780              
  Partials      138      138              

Impacted Files	Coverage Δ
pkg/services/discord/discord_config.go	82.35% <ø> (ø)
pkg/services/discord/discord_json.go	91.66% <ø> (ø)
pkg/services/discord/discord.go	56.52% <100.00%> (+0.96%)	⬆️

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