Add support for running the bot in forks

[canihavesomecoffee]

(Similar to/clone of CCExtractor/ccx_gitbot#8).

Currently, the only way you can get tests run on the sample platform is by opening a PR or committing to the master branch. The latter is only possible for core contributors, so the only option for a non-core contributor is to open a PR, and push to that branch. If you just want to test something out for a fix, this is far from ideal. So the platform needs to be expanded so that:

• A contributor registers on the platform (that's present already)
• Links his GitHub account (present)
• Requests access to use the platform from his fork
• Gets approved/denied by a core contributor
• Then can use the platform.