fix(discord): rename channel into webhook ID

[piksel]

Note: Breaking change for anything explicitly passing a config struct to the discord service (which should normally never be the case).

fixes #156

[codecov]

Codecov Report

Merging #159 (a78dd7c) into next (28298f4) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             next     #159   +/-   ##
=======================================
  Coverage   68.87%   68.87%           
=======================================
  Files          76       76           
  Lines        2262     2262           
=======================================
  Hits         1558     1558           
  Misses        572      572           
  Partials      132      132           

Impacted Files	Coverage Δ
pkg/services/discord/discord.go	55.55% <100.00%> (ø)
pkg/services/discord/discord_config.go	82.35% <100.00%> (ø)

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