Alpha: Surface talk pages on article

Adds a talk speech bubble to right of article title
When clicked opens an overlay showing current discussions
For non-js users points to talk page

Suggested next steps:
Iterate on design
Show intermediate menus for smaller subheadings e.g. h3s
Allow contributions via overlay (I'd suggest a + icon in the
top right corner)
Currently doesn't work with dynamic page loads (we should refactor that
code to use page.js)

Change-Id: I6e206267b754e1851c1223efb249a3830ba963d2
Merge "Alpha: Surface talk pages on article"