Auto-update code

[Alanaktion]

I think we could fairly easily implement an auto-updater for both the main Phproject application and any plugins that are installed, as long as both were installed via git with https. Things are much trickier if the repo was cloned with ssh, since the web server user generally won't have access to a private key tied to a GitHub account (and never should!), but doing updates with https only requires the web server user to have access to the git binary, which I assume is fairly common.