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>

scripts/uigen.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+set -e
+
+origdir=$(pwd)
+tempdir=$(mktemp -d update-ui-assets.XXXXXX)
+ui_commitish=${UI_COMMITISH:-develop}
+
+cd $tempdir
+git clone https://github.com/hashicorp/watchtower-ui
+cd watchtower-ui
+git fetch origin ${ui_commitish}
+git checkout ${ui_commitish}
+
+docker-compose -f docker-compose-embedding.yml run build
+
+cd $origdir
+
+go-bindata -fs -o internal/ui/assets.go -pkg ui -prefix "${origdir}/${tempdir}/watchtower-ui/ui/core/dist" "${origdir}/${tempdir}/watchtower-ui/ui/core/dist"
+
+rm -rf $tempdir