Bug 1032694 - Elements misaligned on new task creation page

I updated the styling of the form to address the issues in the bug, and to use the sandstone style appropriately as per http://ossreleasefeed.github.io/Sandstone/

@glogiotatidis r?
@ossreleasefeed mind taking a look at the finished product on dev [1]?

[1] https://oneanddone-dev.paas.allizom.org/en-US/tasks/edit/4/ 
You can change this macro to automatically set the `required` class like this:

```
{% macro form_field(field) %}
 <div class="field {% if field.field.required %}required{% endif %}">
{{ field.errors }}
{{ field.label_tag() }}
{{ field }}
</div>
```