Trim leading whitespace-only lines

[gaearon]

This adds trimming of leading whitespace-only lines to posting.

The goal is to reduce cases where people accidentally linebreak before their replies.

This doesn't break ASCII art — we don't trim whitespace on leading lines that contain non-whitespace characters.

Test Plan

Read the regex, convince yourself it's correct.

Try cases like this:

 
    

  test
test
  test  
  

Expected output:

  test
test
  test

[github-actions]

Old size	New size	Diff
6.97 MB	6.97 MB	34 B (0.00%)

---

[pfrazee]

Tested the regex, LGTM