Introduce basic mindset UI element

Add a UI element to the top right of
the page that changes the mindset of
the app, which we'll use for changing
the UI of the story list.
Mindset: Add move-to-top chevron to story

It is shown when the 'bump' mindset is
selected. Wired up a click handler as well.
Mindset: Hide UI element from xs screens

It doesn't quite fit on a phone screen.
Let's hide it for now, and perhaps revisit
it later.
Mindset: Move to top, minus DOM

Move the clicked story to the top
in the data on the server. Still
need to reflect the changes
visually.
Mindset: Update the view model when moving to top

Apply our update-view-model code to moving a
story to the top of the list.
Use keyboard to change mindset

Right now:
 * 1 is 'detailed' (move)
 * 2 is 'bump' (chevron)
Mindset: Hide chevron for first story
Mindset: Pulse the story that is moved to top