feat(discord): send multiple messages when exceeding limits

[piksel]

Currently, the discord service sends notifications as one message with several embeds (one for each line). When there are more lines than the number of embeds permitted by the Discord API, they are simply omitted.
This will change that behaviour, so that it will instead send multiple messages when the limit is reached.

Ref containrrr/watchtower#1126

[codecov]

Codecov Report

Merging #215 (a23d156) into main (ef1a21e) will increase coverage by 0.19%.
The diff coverage is 97.43%.

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
+ Coverage   77.54%   77.73%   +0.19%     
==========================================
  Files          95       96       +1     
  Lines        2859     2866       +7     
==========================================
+ Hits         2217     2228      +11     
+ Misses        469      468       -1     
+ Partials      173      170       -3     

Impacted Files	Coverage Δ
pkg/services/discord/discord.go	80.70% <94.44%> (-0.55%)	⬇️
pkg/services/discord/discord_json.go	90.47% <100.00%> (-2.12%)	⬇️
pkg/util/partition_message.go	94.00% <100.00%> (+4.00%)	⬆️
pkg/services/gotify/gotify_json.go	0.00% <0.00%> (ø)
pkg/util/jsonclient/jsonclient.go	71.11% <0.00%> (+0.65%)	⬆️
pkg/services/gotify/gotify_config.go	90.90% <0.00%> (+2.67%)	⬆️
pkg/services/gotify/gotify.go	85.71% <0.00%> (+8.79%)	⬆️

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