Convert all i18n to JSON blobs

Split the i18n messages into four groups:
* oojs-ui (moved to the oojs-ui repo in a separate commit)
* VE core
* VE-MW (MediaWiki-specific things)
* VE-WMF (Wikimedia-specific things)

The VE-WMF group is new, and we'll split WMF-specific code out into
it later, for now it's just messages associated with that code.

Each language has its own JSON file at modules/MODULE/i18n/LANG.json

Kept messages in VisualEditor.i18n.php as the master copy, because
TranslateWiki can't deal with the JSON files just yet. Added a script
to rebuild the JSON files from the PHP file.

Change-Id: I94e084b2f10994f41324fd08a05ff7f8391ea2eb