Add tour for VisualEditor

* It automatically chooses the correct tour depending on whether
  VisualEditor is detected.
* If they click 'Show help' while already editing with VE, it will
  simply show the applicable step (pointing to Save).
* Mark that old tour is specific to wikitext flow, and new one is for
  VE flow.
* Don't preload 'Show help' tour on server.

Bug: 50186
Change-Id: I92958ba665b9462ca4ed941efcbe1170e3a0697c
Merge "Add tour for VisualEditor"