Issue Type: Bug

Enter Review mode for a PR

Type your'e comment (or not) and press "approve".

The button is still enabled and there's no indication that you've already approve this.

The UI should show the user if a PR was already approved/requested changes (possibly like GH does with the green_checkmark / red cross)

And if it was already approve it doesn't make much sense to still allow the user to "approve again"

Extension version: 0.1.4
VS Code version: Code 1.27.1 (5944e81f3c46a3938a82c701f96d7a59b074cfdc, 2018-09-06T09:14:32.385Z)
OS version: Darwin x64 17.7.0