Add product version to question cards for contributors

[denyshon]

Resolves #2958.

To avoid fetching tags, we can use question metadata to get product versions (and display them). However, if we use raw data from the corresponding fields, this would expose us to all kinds of spam and UI breakages related to user input. In order to prevent that, this patch saves our validated version number (used to apply tags) into a separate question metadata field, and then displays it on question cards. For older questions, we fall back to the raw data from the version fields.

The SCSS changes do not represent any visual change, and are purely for clarity/future.

[akatsoulas]

This will be called on every update of the question. The add_metadata will call basically the QuestionMetaData.objects.create(...) but he model has a unique constraint applied. I suspect this is is going to lead to an IntegrityError from the DB.

[akatsoulas]

I wonder if we can push the logic with the nested if statements into python in a @property like display_version to simplify the template to something like

{% if user_is_contributor and question.display_version %}
  {{ question.display_version }}
{% endif %}