"Update Branch" button for stale branches #1950

Closes https://github.com/sindresorhus/refined-github/issues/1236

• It works only when the branches are on the same repo, because of API limitations
• It works even if master was force-pushed
• The name is simplified to master because people get it but it works on any branch

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• Make less prominent, this shouldn't look like a necessary step.

  HELP: Where/how should we show this button? Does it make sense there? This is what I got so far, which doesn't look right (warning icon, too prominent)

• The button might not appear sometimes; I'll fix this once we figure out where to place it 👆

• Only show if necessary, like when a branch is stale or when GH already shows it

  HELP: how do we find out whether a branch is stale? Edit: maybe this https://developer.github.com/v3/repos/commits/#compare-two-commits

• Handle errors. I'll probably change the API to just ignoreHTTPStatus: false instead of accept404. API reference: https://developer.github.com/v3/repos/merging/

• Update readme

cc @rarkins @sindresorhus

<img width="779" alt="" src="https://user-images.githubusercontent.com/1402241/56434750-842de800-6308-11e9-9092-75cd703d4414.png">