Static asset compilation for Consul UI

[ryanuber]

Adds the static web assets into the Consul binary with an explicit flag required to enable.
Thanks to @rgl for prior art in #1274.

[slackpad]

Code LGTM but the build failure looks related to this change:

--- FAIL: TestEnableWebUI (0.13s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x7e904e]
goroutine 6499 [running]:
testing.tRunner.func1(0xc82150c5a0)
    /usr/local/go/src/testing/testing.go:450 +0x171
github.com/elazarl/go-bindata-assetfs.(*AssetFS).Open(0xc821e0bef0, 0xc82164e2b0, 0xf, 0x0, 0x0, 0x0, 0x0)
    /home/travis/gopath/src/github.com/elazarl/go-bindata-assetfs/assetfs.go:148 +0x22e
net/http.serveFile(0x2b9f5b923f80, 0xc821f980a0, 0xc8217a9880, 0x2b9f5b954b88, 0xc821e0bef0, 0xdfd190, 0x1, 0xdfd201)
    /usr/local/go/src/net/http/fs.go:394 +0x5b1
net/http.(*fileHandler).ServeHTTP(0xc821f79860, 0x2b9f5b923f80, 0xc821f980a0, 0xc8217a9880)
    /usr/local/go/src/net/http/fs.go:483 +0x19c
net/http.StripPrefix.func1(0x2b9f5b923f80, 0xc821f980a0, 0xc8217a9880)
    /usr/local/go/src/net/http/server.go:1455 +0x1a5
net/http.HandlerFunc.ServeHTTP(0xc821e0bf20, 0x2b9f5b923f80, 0xc821f980a0, 0xc8217a9880)
    /usr/local/go/src/net/http/server.go:1422 +0x3a
net/http.(*ServeMux).ServeHTTP(0xc821e0bec0, 0x2b9f5b923f80, 0xc821f980a0, 0xc8217a9880)
    /usr/local/go/src/net/http/server.go:1699 +0x17d
github.com/hashicorp/consul/command/agent.TestEnableWebUI.func1(0xc8220967d0)
    /home/travis/gopath/src/github.com/hashicorp/consul/command/agent/http_test.go:642 +0x291
github.com/hashicorp/consul/command/agent.httpTestWithConfig(0xc82150c5a0, 0xc822091f30, 0xf9a880)
    /home/travis/gopath/src/github.com/hashicorp/consul/command/agent/http_test.go:693 +0x132
github.com/hashicorp/consul/command/agent.TestEnableWebUI(0xc82150c5a0)
    /home/travis/gopath/src/github.com/hashicorp/consul/command/agent/http_test.go:650 +0x45
testing.tRunner(0xc82150c5a0, 0x1297db8)
    /usr/local/go/src/testing/testing.go:456 +0x98
created by testing.RunTests
    /usr/local/go/src/testing/testing.go:561 +0x86d

[ryanuber]

Huh... I had checked out go-bindata-assetfs a while back. Upon updating I see this problem locally. Seems related to elazarl/go-bindata-assetfs#23

[ryanuber]

I fixed this, just needed to re-generate the assets, but now I'm getting some really scary test failures from inside the state store and all over the place. I'll try pushing this through travis a few more times and see what we get.

[ryanuber]

Test failures are now unrelated to this so going to merge this in.