* Add default_settings class method to ScopedSettings
ScopedSettings was extended to use value of unscoped setting instead of
only using defaults set in config/settings.yml for selected settings.
This adds possibility for admins to set default values of users' settings,
for example default theme (as requested in #7092).
* Add ability to change an instance default theme
Closes#7092
Previously these returns 302 redirects instead of 403s, which meant posting links to admin pages in slack caused them to unfurl, rather than stay as a link. Additionally, require_admin! doesn't appear to be actively used, on require_staff!
