Email sign-in, admin role, pytest

[osteele]

• Alongside Sign in with Slack, there is also an email-based sign-in
• Users who sign in with email that is listed in an (as-yet undocumented) ADMIN_EMAILS list, are admins
• Switched test runner from unittest to pytest — solving many problems

[codecov[bot]]

Codecov Report

Merging #261 into dev will increase coverage by 6.49%. The diff coverage is 77.68%.

@@            Coverage Diff             @@
##              dev     #261      +/-   ##
==========================================
+ Coverage   68.26%   74.76%   +6.49%     
==========================================
  Files          25       46      +21     
  Lines        1314     2009     +695     
==========================================
+ Hits          897     1502     +605     
- Misses        417      507      +90

Impacted Files	Coverage Δ
tests/test_subscription_resources.py	64.28% <ø> (ø)
abe/document_models/event_documents.py	72.36% <ø> (ø)	:arrow_up:
abe/resource_models/subscription_resources.py	40% <0%> (-5.72%)	:arrow_down:
abe/sample_data.py	90.36% <100%> (ø)	:arrow_up:
tests/test_email_helpers.py	100% <100%> (ø)
abe/resource_models/ics_resources.py	79.41% <100%> (ø)	:arrow_up:
tests/__init__.py	100% <100%> (ø)
tests/test_recurrences.py	100% <100%> (ø)
tests/test_auth.py	100% <100%> (ø)
abe/resource_models/account_resources.py	100% <100%> (ø)	:arrow_up:
... and 42 more

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