Add plan summary to unfolded part of comment

[wkrysmann]

If there are multiple projects planned and markdown folding is enabled user must click on each "Show details" button separately and scroll through the plan to check if there are any changes to the infrastructure. This may be pretty time consuming when multiple projects are planned. Some of them may not have any changes in them, due to the dependency resolution.
This PR adds the extra summary line, so user can quickly eyeball what is the impact of his change for each project without unfolding the output.

[codecov]

Codecov Report

Merging #1518 (67cc3b7) into master (25103d7) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1518      +/-   ##
==========================================
+ Coverage   70.21%   70.23%   +0.02%     
==========================================
  Files          94       94              
  Lines        6530     6535       +5     
==========================================
+ Hits         4585     4590       +5     
  Misses       1555     1555              
  Partials      390      390              

Impacted Files	Coverage Δ
server/events/markdown_renderer.go	90.10% <100.00%> (ø)
server/events/models/models.go	75.00% <100.00%> (+0.89%)	⬆️

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[nishkrishnan]

Nice addition 👍