Feedback form modal in sidebar

[RW + FW]

This pull is ready to review, but not merge.

The functionality is here and good, we're just ironing out a few small styling details with @andibyday as well.
I thought we had a `windowLocation` method in [`Tahi.Utils`](https://github.com/Tahi-project/tahi/blob/master/app/assets/javascripts/utils.js.coffee), but we don't. It's useful so it can be stubbed in tests.