Enable a single-user mode and make it the default

[voxpelli]

It's great that people can use my endpoint, but in the spirit of the indieweb they should of course be able to host it on their own as well.

So it should be possible to activate a single user mode where people can't sign up but where rather everything is configured like it is in https://github.com/voxpelli/webpage-micropub-to-github

It should also be possible to easily deploy the project, using an Heroku button or any of the other buttons that have been suggested for that project: voxpelli/webpage-micropub-to-github#26

Do however note below such a button that unlike the Micropub endpoint a Webmention endpoint will need to pretty much run 24/7 as long as people interacts with oneself 24/7 and with scripts embedded on one sites that's very likely to happen. With a future #37 the need would be less, but still – every mention or Salmention would activate ones server and that would quickly eat up the free server time one has on Heroku