Enable Parsoid's edit caching on all public wikis

This also factors out Parsoid's config out from VisualEditor's. We were
previously setting $wgVisualEditorParsoidURL and the like in the config
for VisualEditor; however, in the Modern World™ other projects might
want a Parsoid instance too, so let's expose it more gracefully.

This thus sets both $wgVisualEditorParsoidURL & $wgFlowParsoidURL to be
set by a single new cluster-wide variable.

$wgVisualEditorParsoidProblemReportURL, un-used for a while, is killed.

Change-Id: Ic0dc9a7316daf34df82e47a8c5e945df5294ac58
Merge "Enable Parsoid's edit caching on all public wikis"