Add a Github CI check to make sure that if an asset is edited, ui/bindata.go is updated

[salemhilal]

We serve static assets from a serialized file in order to make Hound go-get-able (see #7). Confirming that UI PRs come with an update to ui/bindata.go would be helpful for future PRs.

Here's some helpful context (thanks to @kellegous for a lot of this):

1. ui/bindata.go contains all the contents from ui/assets/* and is generated here in the Makefile.
2. Generating ui/bindata.go as part of the install (i.e. in //go:generate) might be finicky on some systems, so sticking with manually updating it is a safer play for now.
3. As mentioned above, Issue #7 has more context for why we use this approach.
4. If you're curious, assets are read in from ui/bindata.go through calls to Asset in ui#serveAsset.