Various updates to UI server

[mateiz]

• Add an mlflow server command separate from mlflow ui that can run with more workers and might eventually provide more production options
• Use gunicorn to run ui and server
• Miscellaneous fixes to CLI option names and their help strings

[aarondav]

LGTM. Ran both locally, worked as expected.

[codecov-io]

Codecov Report

Merging #61 into master will decrease coverage by 0.26%.
The diff coverage is 36.95%.

@@            Coverage Diff            @@
##           master     #61      +/-   ##
=========================================
- Coverage   77.46%   77.2%   -0.27%     
=========================================
  Files          45      45              
  Lines        2592    2610      +18     
=========================================
+ Hits         2008    2015       +7     
- Misses        584     595      +11

Impacted Files	Coverage Δ
mlflow/store/file_store.py	90.95% <0%> (-0.48%)	⬇️
mlflow/experiments.py	57.89% <100%> (ø)	⬆️
mlflow/server/handlers.py	27.9% <12.5%> (+0.02%)	⬆️
mlflow/server/__init__.py	65.21% <37.5%> (-16.04%)	⬇️
mlflow/cli.py	84.5% <81.81%> (+0.92%)	⬆️

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