Make `update-pr-from-base-branch`'s button smaller #2723

Currently the "Update branch" is rather large and it looks like an action that should be taken. It's not really something the user should do that often, so we should probably de-emphasize it.

<table><td> <img width="753" alt="" src="https://user-images.githubusercontent.com/1402241/72798583-fc4d8380-3c75-11ea-8972-08c10b385d34.png"> </table>

This is an alternative, it just replaces .btn with .btn-link:

<table><td> <img width="749" alt="" src="https://user-images.githubusercontent.com/1402241/72798798-67975580-3c76-11ea-8df3-e8ed31ae15bc.png"> </table>

Also:

• it should probably use a window.prompt() window to ask for confirmation
• it could have an "info" icon with a link to https://github.com/sindresorhus/refined-github/issues/1236 or any other post with a clearer explanation of when this action may be useful