Settings UI ready

Note - removed Persona UI from the settings app, rather than leave it
alongside the FXA UI. Merge was otherwise straightforward.

Conflicts:
	apps/settings/index.html
	apps/settings/style/settings.css

Conflicts:
	apps/settings/index.html
WIP commit to get helper feedback
another WIP commit, wiring up the logged-in screen
WIP commit: not reset password, change password.
WIP: l10n seems to work now
Rename menu item from persona to fxa
WIP show fxa state in main settings menu panel
WIP settings fxa unverified state

  This screen is pending final UX, so this is just placeholder code.
fix gjslint errors in js/fxa.js
Add additional states to main panel, refactor JS.
Some overlay tweaks, not done yet
listen for events correctly, remove unimplemented events, tweak spacing
listen for events correctly in fxa_menu
Remove change password--feature not on 1.4 roadmap

  Per @SamPenrose here: https://github.com/mozilla-b2g/gaia/pull/14101/files#r8941176
Playing with code structure + learning from other settings apps.

  Have the UI bits split into separate files, but don't have everything
  wired up quite yet. Good times thoguh.
move everything into one js file
remove empty fxa js subfiles
WIP unification, wiring up DI-style
First pass at using the mvvm/models.js Observable approach.
Update model to use correct event format