Embed static assets

[arl]

Increase the minimum required Go version to go1.16 in order to use go::embed directive on the assets directory. In addition to reducing the number of files in the package, this also greatly simplifies:

• the building process (no additional go generate step)
• the development process (no need for -tags dev)
• the CI (no need to check assets are up to date anymore)

Update README indicating the mini go version but note that it is still possible to use statsviz@v0.0.4 if you're stuck with go1.12+.

[codecov-commenter]

Codecov Report

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