Move setOnWikiMessages() from init.js to WebUtil
In order for the coming translation gadget (see !256 (merged)) to be able to use the same system of message loading, move the function into WebUtil.
This doesn't make any changes, but does move the parseJSON()
function to be within setOnWikiMessages()
so that it's not exposed to anywhere else (in case anyone thinks it's a general-purpose JSON parsing thing).
Bug: T371098