Codebase UI local deployment

[pchiusano]

Discussed with @stew @hojberg and @runarorama how UCM releases will embed the codebase UI. I think this writeup accurately captures the decisions from that, but pipe up if not.

Here's the UX:

The UX is: User starts up UCM which provides a URL to visit. User visits that URL and gets the codebase UI. (Later we might have an electron app which bundles the codebase UI and UCM, however that's out of scope for now.)

Nice touch: the startup message should also say how to find that URL if the user forgets it. Like maybe help ui shows the URL as well.

Here's the plan:

• @runarorama will change the codebase server to serve the API at <token>/api and the <token>/ui. The /ui endpoint will look for a file /ui/index.html by default or will serve whatever file is specified via a --ui=/path/to/dev-ui/index.html flag and/or environment variable. If no such file exists, it will 404 as normal.
• @hojberg will get CI for the codebase-ui project to produce GitHub releases containing the HTML+JS for the built codebase UI. @stew to support this.
• @stew will create a third project, maybe ucm-release, which will pull any two named versions of Unison and the codebase UI from GitHub releases and package them up into a release.

For later, @stew or someone else could work on doing daily or weekly releases, but I don't think that is MDP for M2. You can tag releases as 'daily' and then they don't show up in the main releases page, see for instance this example