Use <base> and config webpack at runtime to allow path prefix

[tiffon]

Fix #42.

Allows a path prefix to be used through four changes:

• package.json#homepage is set to ".". When create-react-app generates the build index.html the static assets are linked via relative paths.
• Added a <base> element to the index.html template. All relative paths (e.g. the static assets referenced in HTML) are relative to the value of <base>, which is set to "/" in index.html, but the query-service can replace it (see Provide a means to define a path-prefix for the UI jaeger#745). It's worth noting, this only affects relative paths.
• The publicPath value in webpack is configured at runtime via the __webpack_public_path__ global. The value is derived from the <base>, and ensures any chunks or other assets (like the logo) are loaded relative to the site prefix.
• The pathPrefix used for fetch() calls is based on the <base> value instead of package.json#homepage

As a side note, this PR can be merged before any work is done in the query-service.

[codecov]

Codecov Report

Merging #198 into master will increase coverage by <.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
+ Coverage   91.46%   91.47%   +<.01%     
==========================================
  Files          95       96       +1     
  Lines        2133     2135       +2     
  Branches      435      436       +1     
==========================================
+ Hits         1951     1953       +2     
  Misses        161      161              
  Partials       21       21

Impacted Files	Coverage Δ
src/index.js	0% <ø> (ø)	⬆️
src/api/jaeger.js	97.22% <ø> (-0.08%)	⬇️
src/utils/prefix-url.js	100% <100%> (ø)	⬆️
src/site-prefix.js	80% <80%> (ø)
...neViewer/TimelineHeaderRow/TimelineViewingLayer.js	92.59% <0%> (+1.85%)	⬆️

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[yurishkuro]

what's the issue with code coverage? the numbers are always confusing does 88.88% of diff hit refer to coverage of the new/changed code?

[tiffon]

what's the issue with code coverage? the numbers are always confusing does 88.88% of diff hit refer to coverage of the new/changed code?

@yurishkuro I think that means of the modified lines 88.88% of them were hit by code coverage.