Flask-Login for user management

[alysivji]

• use Flask Login to enable user sessions
• set up login and logout views.
• set up flask test client to use custom class provided by Flask-Login library

Resources

• https://flask-login.readthedocs.io/en/latest/

[codecov-commenter]

Codecov Report

Merging #278 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   97.26%   97.28%   +0.02%     
==========================================
  Files          68       68              
  Lines        1863     1880      +17     
==========================================
+ Hits         1812     1829      +17     
  Misses         51       51              

Impacted Files	Coverage Δ
busy_beaver/app.py	93.87% <100.00%> (+0.12%)	⬆️
busy_beaver/apps/slack_integration/api/oauth.py	100.00% <100.00%> (ø)
busy_beaver/apps/slack_integration/models.py	100.00% <100.00%> (ø)
..._beaver/apps/slack_integration/oauth/oauth_flow.py	100.00% <100.00%> (ø)
...sy_beaver/apps/slack_integration/oauth/workflow.py	97.05% <100.00%> (-0.05%)	⬇️
busy_beaver/apps/web/views.py	100.00% <100.00%> (ø)
busy_beaver/extensions.py	100.00% <100.00%> (ø)

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