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
refs https://github.com/TryGhost/Ghost/issues/7865
- updates all settings screens to use EC tasks and `gh-task-button` to get save-state save buttons
- removes now-unused `settings-save` mixin
- moves the order of button color CSS so that grey buttons can change to green/red after completing
- removes the heading from `apps-loading` template so that there's no odd flash when loading slack/amp screens directly
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
