[Draft] Python 3.6 Upgrade / AMO Rebase

[MelissaAutumn]

Initial rebase of AMO (tag:2019.02.28) to ATN master:

• Don't generate Firefox/Android addons for local dev
• Set Thunderbird as the default app
• Only enable Firefox/Android if in tests
• Add auth bypass for development
• Adjusted legacy extension upload message for ATN
• Add contextmanager function to hack around test fixtures and addons-linter woes
• Update ci workflow
• Lots of test fixes

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This is a rebase up to the 2019.02.28 tag on AMO, right before they removed the django front-end. It's probably a good base to then upgrade as we need it. Tests are currently failing due to some category shenanigans (overlapping ids, changed ids, etc...) and some other minor issues. But the only thing failing on the web-side is the addon upload (due to the category issue.)

I'm sure there were Thunderbird specific tweaks I'll have to go back and fix though.

[github-actions[bot]]

Python 3.6 Test Results

5 500 tests   5 322 :white_check_mark:  21m 27s :stopwatch:     1 suites     35 :zzz:     1 files      143 :x:

For more details on these failures, see this check.

Results for commit 9e851062.

:recycle: This comment has been updated with latest results.