CSS styles are conflicting with Django Admin

When loading martor in the admin, it loads CSS rules which overrides many admin styles. For example, when using bootstrap theme, it causes collapsible fieldsets to be completely hidden:

```css
.collapse:not(.show) {
display: none;
}
```

Even when using the "semantic" theme, there are many style definitions that are not namespaced, and causes default admin styles to be overriden, for example, `h2` headings look way to big because of this rule in `martor-admin.css`:

```css
h2 {
font-size: 16px!important;
margin: 1em 0 0.5em 0!important;
}
```

I'm assuming these style definitions are only meant for the preview. If that's the case, then they should be namespaced to the preview box.