fix(slack): limit attachments to 100

[piksel]

also removes last attachment if text is empty

fixes #238

[codecov]

Codecov Report

Merging #241 (4ddab62) into main (6dbcb12) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #241      +/-   ##
==========================================
+ Coverage   77.07%   77.11%   +0.04%     
==========================================
  Files          95       95              
  Lines        2844     2849       +5     
==========================================
+ Hits         2192     2197       +5     
  Misses        473      473              
  Partials      179      179              

Impacted Files	Coverage Δ
pkg/services/slack/slack_json.go	100.00% <100.00%> (ø)

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[justinsteven]

I'm sorry but I'm not totally familiar with slack's structure and the meaning of attachments. Is it required that attachments be used? I'm guessing it's what's responsible for the pill-shaped graphics beside each line:

Perhaps sending more than 100 lines at once isn't a common thing, but having 99 one-line attachments and then a multi-line attachment could look odd.

Basically, do we need to be using attachments, or can we do without them.

[piksel]

It's mainly to allow color support, so that you can set the color of the "pill" in front of the text to show errors with red etc.
For some reason, attachments is the only API that still allows that to be specified. We could add another configuration option to send the messages as text instead, but that would instead be limited to the message length limit. Or use the blocks API, which probably have other limits...