closes#9060
- Update `gscan` - it now extracts custom templates and exposes them to Ghost
- Add `custom_template` field to post schema w/ 1.13 migration
- Return `templates` array for the active theme in `/themes/` requests
- Users with Author/Editor roles can now request `/themes/`
- Front-end will render `custom_template` for posts if it exists, template priority is now:
1. `post/page-{{slug}}.hbs`
2. `{{custom_template}}.hbs`
3. `post/page.hbs`
refs #8126, #8221, #8223✨ New 'Proxy' for all helper requires
- this is not currently enforced, but could be, much like apps
- the proxy object is HUGE
- changed date to use SafeString, this should have been there anyway
- use the proxy for all helpers, including those in apps 😁✨🎨 Single instance of hbs for theme + for errors
- we now have theme/engine instead of requiring express-hbs everywhere
- only error-handler still also requires express-hbs, this is so that we can render errors without extra crud
- TODO: remove the asset helper after #8126 IF it is not needed, or else remove the TODO
🎨 Cleanup visibility utils
🎨 Clean up the proxy a little bit
🚨 Unskip test as it now works!
🎨 Minor amends as per comments
