Truncate extension descriptions

This is for #8216.

- [x] Code to toggle between original and truncated description
- [x] Visual review
- [ ] Code review
This is fine, but maybe a some indentation will make the code look better like:
```
            {{#hasShortDescription}}
                {{metadata.shortdescription}}
            {{/hasShortDescription}}
            {{^hasShortDescription}}
                {{#metadata.description}}
                    {{metadata.description}}
                {{/metadata.description}}
                {{^metadata.description}}
                    <p class="muted"><em>{{Strings.EXTENSION_NO_DESCRIPTION}}</em></p>
                {{/metadata.description}}
            {{/hasShortDescription}}
```