Remove dependency of go-bindata (#818)

* Use new embed functionality for serving UI assets

* Use new embed functionality for serving resources. Remove dependency on go-bindata

* Remove Go 1.15

ui/embed.go

@@ -0,0 +1,14 @@
+package ui
+
+import (
+	"embed"
+	"io/fs"
+)
+
+//go:embed build/*
+var filesystem embed.FS
+
+func Assets() fs.FS {
+	build, _ := fs.Sub(filesystem, "build")
+	return build
+}