Changes to go along with a new, more flexible schema

[thcrock]

• Change unit_id and unit_score to entity_id and score
• Add as_of_date to evaluations and predictions
• Change MetricOptions to include dropdown populated from absolute value/percent thresholding defined by props
• Seed MetricOptions props using YAML config
• Pass as_of_date to different graphs that need it to scope data correctly
• Use prettifying functions to convert metric parameters between database and UI representations

[codecov-io]

Codecov Report

Merging #34 into master will increase coverage by 2.62%. The diff coverage is 84%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   71.01%   73.64%   +2.62%     
==========================================
  Files           4        4              
  Lines         207      239      +32     
==========================================
+ Hits          147      176      +29     
- Misses         60       63       +3

Impacted Files	Coverage Δ
webapp/controller.py	67.69% <82.85%> (+4.96%)	:white_check_mark:
webapp/query.py	81.53% <86.66%> (+0.4%)	:white_check_mark:

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