Add Codecov reporting.

[jezdez]

This also restructures the way the Circle config calls the tests a bit by extending the bin/run script.

Pull Request checklist

• Documentation: If this feature has or requires documentation, the relevant docs have been updated.
• Changelog: This PR updates the changelog with any user-visible changes.
• Tests: This PR includes thorough tests or an explanation of why it does not

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7081094). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #2322   +/-   ##
=========================================
  Coverage          ?   69.36%           
=========================================
  Files             ?      220           
  Lines             ?     5331           
  Branches          ?      853           
=========================================
  Hits              ?     3698           
  Misses            ?     1605           
  Partials          ?       28

Impacted Files	Coverage Δ
server/tests/plugins/staticfiles.py	87.5% <ø> (ø)
server/tests/plugins/lean.py	77.77% <100%> (ø)

---

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 7081094...08d735d. Read the comment docs.

[wlach]

lgtm minus the changelog issue