Serve internal resources using FileResourceServer ensuring path is standardized.

[kilnerm]

Revert changes to serve internal resources using static file server.
Ensure serving path are standardized.
Add further tests to check serving boundaries.

[codecov-io]

Codecov Report

Merging #1278 into master will increase coverage by 0.82%.
The diff coverage is 83.96%.

@@            Coverage Diff             @@
##           master    #1278      +/-   ##
==========================================
+ Coverage   88.14%   88.97%   +0.82%     
==========================================
  Files          38       39       +1     
  Lines        2404     2412       +8     
==========================================
+ Hits         2119     2146      +27     
+ Misses        285      266      -19

Flag	Coverage Δ
#Kitura	88.97% <83.96%> (+0.82%)	⬆️

Impacted Files	Coverage Δ
...ces/Kitura/staticFileServer/StaticFileServer.swift	94.73% <100%> (+9.55%)	⬆️
Sources/Kitura/staticFileServer/FileServer.swift	91.25% <77.77%> (+8.07%)	⬆️
Sources/Kitura/Router.swift	83.87% <82.35%> (+0.27%)	⬆️
Sources/Kitura/FileResourceServer.swift	85.71% <85.71%> (ø)

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