Setting fork to auto-sync with upstream

[stuartpb]

As described in #121 (comment) and #121 (comment), it would be nice to have some kind of mechanism so a fork could be set to automatically be kept in sync with its upstream (whether it be marking a fork as 'read-only' as described in the first aforementioned comment, or only fast-forwarding branches that would not create merge conflicts when "sync upstream" is set as described in the second comment, or something else). This is one of the features @jessfraz was just tweeting about running out of GitHub API calls to implement for herself: https://twitter.com/jessfraz/status/930154061869002752

[stuartpb]

Note that https://upriver.github.io has been established to implement automatic fast-forwarding functionality via the GitHub API.

[acrespo]

I'd LOVE this feature!

[alexivkin]

I built a similar solution to upriver, but it picks the right origin automatically and lets you see how far ahead or behind your fork is - https://forkrefresh.herokuapp.com

[styfle]

Possible duplicate of #121