Bug 50989: Add view frontend to the JSON page.

Instead of showing the raw JSON block when viewing the page
containing the JSON, format the metadata and contents into
sections and display them in a list. This is disabled by
default, but can be enabled by adding
$wgBookManagerv2JsonFrontend = true;
to LocalSettings.

Bug: 50989
Change-Id: Ic30a8012e21be61b1245cc4c1fe08aa77a826e5d