MYLOC-86 - Displaying a cold start message

Involves:
- refactoring the dialog out into a new component
- updating user_locations to use the component
- updating the primary controller to display a dialog when required
- setting a cookie to remember when users have said 'no'

The conditions under which the message are displayed are tested in shouldColdStartDialogBeDisplayed(). 
I created a core js cookie module to support this. We should probably use the cookies.isSupported method in order to decide if the prefer 'star' is displayed to people.
Good spot. At one point I included 'this.preferredLocation.isValidLocation' within shouldColdStartDialogBeDisplayed() but took it out again as they are separate concerns.