Fix login issues on py3 and recent chrome

[tardyp]

This fixes a few issues at the same time

• e2e tests were sometimes broken on the cloud because xvfb screen was smaller than what is tested locally, clicked elements were not always visible on screen.

• people reported issues with basic auth. Looks like this is because of a new CORS restriction enforced by new browsers. For this reason, we should release quite soon because people will see the issue more and more thanks to progressive update of browsers.

• because of the urgency, I backported the twisted fix as a monkey patch for py3, so that people can continue to test py3 with auth enabled. The fix is tested with e2e py3 but coverage is not enabled for those tests.

• e2e tests now does a basic authentication. @desurd can now finish its wider authz e2e tests.

[mention-bot]

@tardyp, thanks for your PR! By analyzing the history of the files in this pull request, we identified @desurd to be a potential reviewer.

[codecov]

Codecov Report

Merging #3145 into master will decrease coverage by 0.03%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master    #3145      +/-   ##
==========================================
- Coverage   88.06%   88.03%   -0.04%     
==========================================
  Files         314      315       +1     
  Lines       33047    33081      +34     
==========================================
+ Hits        29102    29122      +20     
- Misses       3945     3959      +14

Impacted Files	Coverage Δ
master/buildbot/monkeypatches/__init__.py	87.09% <100%> (+1.38%)	⬆️
master/buildbot/www/auth.py	95.61% <100%> (+0.24%)	⬆️
master/buildbot/monkeypatches/twisted9127.py	36.36% <36.36%> (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 d1ba73c...dc01099. Read the comment docs.