No longer escape HTML-safe text given to the icon helper

When using the `icon` helper any argument would be HTML-escaped, even if it was already safe:

```ruby
icon(:pencil, content_tag(:span, 'foo'))
```
```html
<i class="fa fa-pencil"></i> &lt;span&gt;foo&lt;/span&gt;
```

The attached changes fix that.

```ruby
icon(:pencil, content_tag(:span, 'foo'))
```
```html
<i class="fa fa-pencil"></i> <span>foo</span>
```

It's also not necessary to return `html.html_safe` -- rails_xss already takes care of everything, so we now just return `html`.