Fixes to sidebar extension load on startup.

+ Various fixes:
  - better logout behavior in controller extension
  - closing options page for google docs extension
    on startup
  - load history for messages on chat button
    clicked event
  - changed images for messages and notifications
    extensions
  - centering horizontally for promo page
  - fixed bitpop logo on sync_overlay page
  - link to docs viewer extension now points to
    chrome extension web store