We build attributes to pass to RB.FileAttachment when building the

page model data (in {% reviewable_page_model_data %}) and when

rendering the change description for any changed file attachments.

This code was duplicated. The former used Python to generate the data.

The latter used a template (and had a typo, resulting in a bad

filename).
This change consolidates this by adding a method to

FileAttachmentsField that returns a dictionary of attribute data. Both

the page model data and the change description rendering make use of

this, ensuring we have a single place to make changes.
This also speeds up rendering of change descriptions by simplifying that

template and avoiding all the extra jsescape calls.

Unit tests passed.
Manually tested pages with file attachments and change descriptions.