Rename $WebExternalAuth and friends to reduce confusion, and be more specific

In order to reduce confusion with the common extension
RT::Authen::ExternalAuth, the following options have been renamed:

  WebExternalAuth           => WebRemoteUserAuth
  WebExternalAuthContinuous => WebRemoteUserContinuous
  WebFallbackToInternalAuth => WebFallbackToRTLogin
  WebExternalGecos          => WebRemoteUserGecos
  WebExternalAuto           => WebRemoteUserAutocreate
  AutoCreate                => UserAutocreateDefaultsOnLogin

This also makes their utility more evident.
Merge branch '4.2/rename-external-auth'

Conflicts:
	docs/UPGRADING-4.2
	lib/RT/Config.pm