Add basic web interface

[wvengen]

Scrapyd has a basic web interface. It would be useful to have something here too, for easy access. Granted, you can already inspect everything with docker or kubectl, but a basic web interface still has its uses.

The idea is to have a simple interface:

• show projects, versions, spiders
• list jobs
• schedule and cancel jobs
• view job logs (after #12)

[wvengen]

This can be very very simple. SInce there is an API already, it does make sense to make use of it. This means it would be a basic client-side application accessing the API. (Or add JSON and HTML responses to the API endpoints, if you can do that without polluting the API code, that may be an option. Don't over-engineer here.) Don't use some large framework, but keep it small and simple.