Flask: refactor for clarity, then regularly poll slurm for job status

[kevinlul]

Closes #1135

[codecov-commenter]

Codecov Report

Merging #1150 (610c951) into master (d02010b) will decrease coverage by 0.43%.
The diff coverage is 60.97%.

@@            Coverage Diff             @@
##           master    #1150      +/-   ##
==========================================
- Coverage   69.24%   68.81%   -0.44%     
==========================================
  Files          25       26       +1     
  Lines        3239     3290      +51     
==========================================
+ Hits         2243     2264      +21     
- Misses        996     1026      +30     

Flag	Coverage Δ
pytest	68.81% <60.97%> (-0.44%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flask/app/madmin.py	97.56% <ø> (-0.03%)	⬇️
flask/app/schemas.py	100.00% <ø> (ø)
flask/app/slurm.py	22.44% <15.38%> (-15.65%)	⬇️
flask/app/email.py	25.86% <20.37%> (-6.06%)	⬇️
flask/app/manage.py	19.84% <40.00%> (-0.95%)	⬇️
flask/app/blueprints/misc.py	69.93% <50.00%> (ø)
flask/app/login.py	56.41% <56.41%> (ø)
flask/app/stager.py	81.81% <81.81%> (ø)
flask/app/__init__.py	53.57% <100.00%> (-17.86%)	⬇️
flask/app/blueprints/__init__.py	100.00% <100.00%> (ø)
... and 16 more

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[kevinlul]

Tomorrow, this will be broken up into several PRs:

• timezone + configuration refactor
• blueprints subdirectory
• StagerLoginManager
• Stager Flask app subclass
• Mailer class
• Scheduler initialization + poll_slurm implementation

[kevinlul]

Broken up into #1151 #1152 #1153 #1154 #1155