Track undo/redo events

[chenba]

Fixes #4735

[codecov-io]

Codecov Report

Merging #4761 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #4761      +/-   ##
==========================================
- Coverage    41.7%   41.66%   -0.05%     
==========================================
  Files          43       43              
  Lines        1899     1901       +2     
  Branches      353      353              
==========================================
  Hits          792      792              
- Misses       1107     1109       +2

Impacted Files	Coverage Δ
server/src/pages/shot/editor.js	7.17% <0%> (-0.08%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b88810c...6280cdc. Read the comment docs.

[jaredhirsch]

Please add the new events to https://github.com/mozilla-services/screenshots/blob/master/docs/METRICS.md

Added the events to the metrics doc. (Also added the ones for the text tool.) Thanks!

[jaredhirsch]

LGTM, thanks for catching the text tool docs as well