[FEATURE] Labels for range picker

[strangedev]

When presenting survey items to the DataSubject, it is important to communicate what the lower and upper bounds of the range stand for.

The range picker in the front end now supports a startLabel and endLabel and I've modified the API and back end model to accommodate for these changes.

Future feature: Modify all labels in a Dimension at once in the front end.

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This change is 

[codecov[bot]]

Codecov Report

Merging #42 into develop will decrease coverage by <.01%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           develop     #42      +/-   ##
==========================================
- Coverage     0.21%   0.21%   -0.01%     
==========================================
  Files           54      54              
  Lines         1398    1404       +6     
  Branches       124     124              
==========================================
  Hits             3       3              
- Misses        1271    1277       +6     
  Partials       124     124

Impacted Files	Coverage Δ
frontend/src/api/Dimension.js	0% <ø> (ø)	:arrow_up:
frontend/src/api/Question.js	0% <ø> (ø)	:arrow_up:
frontend/src/api/Util/Parse.js	0% <ø> (ø)	:arrow_up:
frontend/src/model/SurveyBase/Config/Range.js	0% <0%> (ø)	:arrow_up:

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