Hosted Mender region selector

With the launch of the European Hosted Mender instance, we need to make sure customers sign up in the right instance. To make it possible, we want to modify the log in and sign up pages in Hosted Mender to make it evident that they belong to a specific region (EU vs. US).

This means the page should have a flag, image or banner stating the region it belongs to (https://hosted.mender.io for the US, https://eu.hosted.mender.io for the EU). It should be possible for the users to move between the two instances with a button, a dropdown or another kind of region selector.

As the two instances are fully independent, there is no need to move data across the two pages. All we need is to redirect the user to the right signup/login URLs.