Adding a frontend for the service.

[haz]

Basically a stripped down port of the existing solver.planning.domains landing page. We can populate it with more info later.

• Seems to work with the docker-compose, and exposes on the regular http port (80)
• Will need some further config to make it https-compatible, I reckon
• Added an error throw-back if anyone hits the /solve endpoint with a POST request

Once we have the server with this frontend, we can proceed with updating the DNS to use solver.planning.domains, and get rid of the legacy heroku one.

[haz]

@nirlipo Any chance you could have a gander?