[Alpha] Talk: Write the number of talk topics in the talk bubble

This gives a sense of the underlying talk page without the user
having to go to the talk page.

Every time talk page is saved, save a property page_top_level_section_count
storing the number of topics

Change-Id: I4833a35b121eb4cd1f2e857079cadea6146d30c8
Merge "[Alpha] Talk: Write the number of talk topics in the talk bubble"