bug 1544084: revert atomic edits

[escattone]

It appears that #5348 has broken editing (see mdn/sprints#1366). I'm not sure why, since I can't reproduce it locally or even on stage, so this PR reverts the change that wrapped the edit view in an atomic transaction, in order to restore functionality for the writers while this is investigated further.

[codecov-io]

Codecov Report

Merging #5366 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5366      +/-   ##
==========================================
- Coverage   95.36%   95.36%   -0.01%     
==========================================
  Files         291      291              
  Lines       25110    25108       -2     
  Branches     1826     1826              
==========================================
- Hits        23947    23945       -2     
  Misses        932      932              
  Partials      231      231

Impacted Files	Coverage Δ
kuma/wiki/views/edit.py	70.13% <ø> (-0.41%)	⬇️

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[peterbe]

It's pretty depressing that the tests didn't break as of this mini-custom-revert. I understand why but it's still depressing. Mocking sucks.

By the way, as of that change, I made it so that send_first_edit_email.delay(new_rev.pk) gets called at the end of the transaction. transaction.on_commit still occurs when your view isn't made atomic but it's just that it gets called no matter what happens. Even if something fails later in the view code.