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
(re #6087)
- some were able to do a different approach
(validations/almost_junction, and settings/*)
- some were replaced with custom speedy cloners
(in orthogonalize clonePoints and osm.js cloneNoteCache)
- some just replaced with JSON.parse(JSON.stringify()))
- Makes the "Sidebar" toggle button permanent but removes the label
- Did some things to the "Save" button to make it the same width whether there
is a count or not (prevents the buttons from jumping when pressing undo/redo)
- Removes a lot of the floated col rules that aren't used much anymore
