Utilize builtin go "embed" package to embed all static files needed by internal viewer UI

[mapl]

dist folder not included in source

[ARolek]

I'm a big fan of this PR, but we're in a peculiar situation where we need to keep supporting the old way of embedding assets. In an ideal situation, we could use go build flags to sniff the version of Go that's being used for building and use the embed package when the version is 1.16 or greater.

cc @gdey

[ARolek]

This looks good to me. No real comments. @gdey any feedback? You have more experience with embed.

[gdey]

Need to look at the build failure. We may need to move the build of the assets around; seems to be failing to find the dist dir according to the CI. (This is when it's running the tests; so we may need to just include the dist dire and a file)

[gdey]

Then shouldn't this be !noViewer,go1.16?

[mapl]

Good catch.
I have now completely omitted the +built constraint in embed.go
It doesn't seem to matter because "embed.go" is only referred in "viewer_embed.go" which is only considered when built with go 1.16+
It compiled fine with no tag set and with -tag NoViewer on Go1.15.14.

[coveralls]

Pull Request Test Coverage Report for Build 6dfae95f3-PR-785

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.05%) to 45.0%

---

Totals
Change from base Build 126d51113:	-0.05%
Covered Lines:	5427
Relevant Lines:	12060

---

💛 - Coveralls

[ARolek]

@mapl can you squash these commits when you have a chance? this is just about ready to be merged.

[mapl]

I made some git mistakes and decided to open another PR for this.