Clean up VisualEditor config

* Pull list of VE-enabled wikis from visualeditor.dblist
* Set $wgVisualEditorParsoidPrefix to $wgDBname rather than a per-wiki value
** This requires a config change for Parsoid to recognize these prefixes

Bug: 47557
Change-Id: I621e98da3be4f787e294d97b350b92d217f43a00