Make update-pr-from-base-branch's button smaller

[fregante]

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.

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

Also:

• it should probably use a window.prompt() window to ask for confirmation
• it could have an "info" icon with a link to "Update Branch" button for stale branches #1236 or any other post with a clearer explanation of when this action may be useful

[fregante]

Keep in mind: drafts have a different copy.

Maybe it can be added as:

Draft pull requests cannot be merged. You can update the base branch.