Allow github editing links on RTD

* Use FontAwesome icons
* Enable Edit link if on RTD
* Pull Source/Edit links right but keep left of search
* Work around broken RTD layout.html
  * Switch off display_github - to prevent RTD displaying its links
  * Rather use READTHEDOCS to switch on alternate RTD behaviour for now
  * Use our own block for source links so that we win in the display
    race.