Update appearance of web UI, and advertise more widely

[tripu]

The web UI of Echidna has been out there for quite some time, but it's not advertised much. I suspect almost no user or group knows about it (although there's an oblique little reference to it on the README). In fact, I've seen at least one editor writing their own simple HTML form just to submit publication jobs to Echidna and check their statuses easily.

I think we should revamp the look & feel of that page a little bit (just enough to be consistent with the UI of Specberus; it should not take long), and start reminding users that that exists.

[tripu]

Three strategies about this (not necessarily exclusive):

• https://labs.w3.org/echidna/ui
• https://github.com/features/actions
• https://travis-ci.com/