Fix pre plan output being removed.

[lkysow]

Prior to this change, we were formatting the output from terraform plan
inside the markdown_renderer. Part of this formatting is to remove all
output relating to refreshing state. The way we were doing this was to
simply delete all lines before the refreshing completes.

The problem with this approach was that any output from custom steps
that ran before plan would also be deleted. This change moves the
formatting of the plan output inside the plan step. Then the
markdown_renderer deals with the pre-formatted output and doesn't need
to delete anything before it combines all the step outputs into one.

[codecov]

Codecov Report

Merging #367 into master will decrease coverage by 0.03%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master     #367      +/-   ##
==========================================
- Coverage   70.54%   70.51%   -0.04%     
==========================================
  Files          61       61              
  Lines        3718     3720       +2     
==========================================
  Hits         2623     2623              
- Misses        908      909       +1     
- Partials      187      188       +1

Impacted Files	Coverage Δ
server/events/markdown_renderer.go	91.89% <ø> (-0.88%)	⬇️
server/events/runtime/plan_step_runner.go	91.54% <83.33%> (-1.79%)	⬇️

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