Implements:
* Gadget definition content and content handler
* Basic validation for gadget definition content
* GadgetDefinitionNamespace implementation of GadgetRepo
* DataUpdates upon editing/deletion of Gadget definition pages
* EditFilterMerged hook for improved error messages
* 'GadgetsRepoClass' option to switch GadgetRepo implementation used
* Lazy-load the GadgetResourceLoaderModule class so we don't need to
load each individual gadget object unless its needed
Note that Special:Gadgets's export feature intentionally doesn't work
yet, and will be fixed in a follow up patch.
Bug: T106177
Change-Id: Ib11db5fb0f7b46793bfa956cf1367f1dc1059b1c
