refs https://github.com/TryGhost/Ghost/issues/9060
- add `{{gh-psm-template-select}}` component
- allows selection of a custom template for a post if the active theme has custom templates
- loads themes on render, only hitting the server if not already in the store
- disables select if post slug matches a `post-*.hbs` or `page-*.hbs` template
- adds `customTemplate` attr to `Post` model
- adds `templates` attr to `Theme` model with CPs to pull out custom vs post/page override templates
- add `.gh-select.disabled` styles to make disabled selects look visually disabled
closesTryGhost/Ghost#8222
- there are fatal and normal errors
- fatal === can't activate a theme
- the normal errors are only returned in development mode (!)
- Separate between `fatal` and normal errors and group them
closes https://github.com/TryGhost/Ghost/issues/8127
- update theme activation to manually set other themes to `active: false` in the store now that we only the active theme back from `/themes/:name/activate` endpoint
- move theme warning list item rendering into `{{gh-theme-error-li error=x}}`
- add `theme-warnings` modal that accepts a warnings list, title, and optional message
- after activating a theme, check if the theme has any warnings or errors and display an appropriate modal informing the user
requires https://github.com/TryGhost/Ghost/pull/8093
- adds `theme.activate()` method and associated adapter method for activating themes rather than relying on `settings.activeTheme`
- minor refactors to the `modals/upload-theme` component to use a full theme model
