Feature/card template preview revised

This PR removes the `<input>` boxes from the *Add a Course* template preview. The inputs could be a source of confusion since it's just a preview, and the user is not meant to input anything, even if the inputs are disabled. I removed the inputs and replaced them with just plain text.

We discussed using some kind of sample card such that the preview would show:

```
LABEL: {SAMPLE VALUE}
```

Currently it just shows static text for the `{SAMPLE VALUE}`.