Use extension.json manifest version 2

[jamesmontalvo3]

extension.json manifest version 2 allows supplying additional information about extension configuration. For example, manifest version 1 may look like this:

	"config": {
		"MyExtSetting": true
	},

But version 2 allows:

	"config": {
		"MyExtSetting": {
			"value": true,
			"path": false,
			"description": "The description for the configuration",
			"descriptionmsg": "myextension-config-myextsetting",
			"public": true
		},
	},

Most interesting there seems "descriptionmsg" which would force us to document the purpose of extension variables, and would make them internationalizable.

Ref: https://www.mediawiki.org/wiki/Manual:Extension.json/Schema#Changes_in_manifest_version_2