Github webhook integration

[glassresistor]

My main issue right now with realms-wiki is that it has no built in tools for merging its repo with a remote github repo or others.

Setting it up to push to github is easy just by adding a post commit hook to the local repo but pushing changes on github to realms is much less easy.

One way to solve this would simply be to add a url to realms-wiki which when hit causes it to run pull on the repository. Since realms is already setup to work with git and run a webserver it should be much easier to add this feature into itself rather than having the user have to strap it on externally.

[gazpachoking]

I haven't gotten back to it yet, but had a quick start on something like this in #155