Visual design tweaks.

* Make the onboarding CTA to 510px wide on pages in the main namespace
  and 365px on pages outside of the main and special namespaces
* Make the "Fix pages that need easy fixes" button 210px wide and remove
  its text shadow
* Reduce the font size of the "No thanks, maybe later" link, which
  closes the onboarding CTA, to 80%

Change-Id: I80df8f918dc563270eda5b0c203bb0c994591dac
Merge "Visual design tweaks."