Add Flow to extension list for message cache.

Move Flow from wmf-config/extension-list-labs to the main
wmf-config/extension-list, in order to generate
ExtensionMessages-1.23wmfNN containing Flow for l10n in advance of
flipping the switch to enable Flow.

Change-Id: If6b6e282ce1529ee1743d6d3e80e1e63e0f37b41
Merge "Add Flow to extension list for message cache."