Global write button on reader pages

[davemart-in]

On P2 we decided to try adding sticky headers to the reader sections (just like the sites screen) and we also decided to add a "Write a post" button.

We probably don't need the dropdown, but we will want to take them to the Calypso site selection screen if they click this button and have more than one site.

Let's make sure we add a tracks and MC stat for this button so we can see how many people are clicking it.

[davemart-in]

We're discussing this issue here: pfsHM7-1e3-p2