Adopt new alert styles / colors

This converts Asimov alerts to the new alert style currently in handover, and @dannymidnight's full screen viewer. Default alerts now include:

```
.alert--success
.alert--error
.alert--caution
.alert--info
.alert--admin
```

The only thing that makes this a breaking change is a default bottom margin (gutter) provided to alerts.

Depends on https://github.com/99designs/asimov-platform-settings/pull/7/files

/cc @andrewk @xzyfer @dannymidnight @dennisroethig