refs https://github.com/TryGhost/Team/issues/755
reqs https://github.com/TryGhost/Ghost/pull/13006
- updated settings model to match new server-side settings
- updated email customisation modal behind the dev experiments flag
- adjusted layout to move to a fixed top bar and scrollable sidebar
- added image upload, toggles and selections for new settings
- updated preview to match selected settings
Co-authored-by: Sanne de Vries <sannedv@protonmail.com>
