WIP: Option to authorize registrations from a file in the package repo

[nkottary]

This lays down the ground work for the plan that is still under discussion in #329. This does not change the existing authorization behaviour of the WebUI so existing installations will not be affected. The file based authorization is enabled by a configuration parameter which is optional.

I've written down the details in the dev docs. We need to update the README.md after this goes live on juliahub.com since users refer to that for help on using Registrator.

[StefanKarpinski]

One comment, let's not call this authorised_registrars.txt, which imo is a bit of an odd name. If this file is going to exist in a lot of repos it seems good to match existing conventions for similar files — how about MAINTAINERS to match the CODEOWNERS file name convention? We should also figure out the file format.