Add mechanism for bundling UI assets. (#153)

* Add mechanism for bundling UI assets.

The scripts/uigen.sh script will check out the UI repo, build assets,
create a static assets file, and clean up.

The dev Makefile target has been modified to run this script if no
assets file is found. There is also a build-ui target that will
unilaterally update (the dev target only builds if the file isn't found,
but doesn't update).

The handler has been updated to add the default org ID as a header if
the request path is "/".

Co-authored-by: Randall Morey <randall@randallmorey.com>

internal/ui/passthrough.go

@@ -0,0 +1,23 @@
+package ui
+
+import (
+	"net/http"
+	"path/filepath"
+
+	"github.com/hashicorp/go-hclog"
+)
+
+func DevPassthroughHandler(logger hclog.Logger, passthroughDir string) http.Handler {
+	// Panic may not be ideal but this is never a production call and it'll
+	// panic on startup. We could also just change the function to return
+	// an error.
+	abs, err := filepath.Abs(passthroughDir)
+	if err != nil {
+		panic(err)
+	}
+	logger.Warn("serving passthrough files at /", "path", abs)
+	fs := http.FileServer(http.Dir(abs))
+	prefixHandler := http.StripPrefix("/", fs)
+
+	return prefixHandler
+}