Refine TODO App example

[vkostyukov]

I was playing around the idea of doing better job with our examples to make them more appealing. This PR adds an HTML frontend for a Finch app. It looks like this.

Additionally this:

• Introduces Endpoint.resource to serve static content (see Introduce asset endpoints #1017)
• Makes use of Ref instead of synchronized
• Adds EndpointResult.output as a syntax (it's a bummer variance doesn't give us another option; I will file a ticket - EndpointResult.output #1021)

Curious to hear what people think about this.

[codecov-io]

Codecov Report

Merging #1020 into master will increase coverage by 0.05%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1020      +/-   ##
==========================================
+ Coverage   84.08%   84.13%   +0.05%     
==========================================
  Files          49       49              
  Lines         949      952       +3     
  Branches       61       59       -2     
==========================================
+ Hits          798      801       +3     
  Misses        151      151

Impacted Files	Coverage Δ
core/src/main/scala/io/finch/EndpointResult.scala	80.95% <66.66%> (-2.39%)	⬇️
core/src/main/scala/io/finch/Endpoint.scala	72.89% <0%> (+0.6%)	⬆️

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[sergeykolbasov]

I'd play with fonts, but otherwise, it's good 😄

👍

[sergeykolbasov]

Unused import?