Consolidate localization behavior and init to a coreLocalizer function and singleton
Explicitly support `en-US` locale
Rename coreData to coreFileFetcher and export a singleton rather than using a property of coreContext
Add `apiConnections` property of coreContext to simplify adding a source switcher
Replace some init functions with re-callable, promise-supporting `ensureLoaded` functions
Make coreContext itself load the UI if a container has been specified at init time
Fix code tests
- ES6ify (now using class syntax to define QAItem objects)
- Fix bug with KeepRight marker rendering not updating properly
- Use `qa-` prefix for the UI element classes to differentiate from iD
validation error related UI element classes
- Move away from "error" where possible in source
- Move away from snake_case naming where possible
Note that some function/method names have been untouched to make life
easier for v3 development. Have added note comments where appropriate.
