Profile select ux changes

This makes the UX around choosing either a manual profile or an authenticated profile a lot better:

![Profile selection dropdown](http://uploads.yoast.nl/profile-select_1A39E13F.png)

![Profile select when manual is checked](http://uploads.yoast.nl/profile-select-manual_1A39E158.png)

Also adds "Select a profile" text and an empty option so we don't default to the first option in the profiles list.