Translate html_attachment view

Set locale in the controller based on the attachment locale, and add
translation keys to the view
Add new translation keys to all locales
Enable right-to-left CSS for html_attachments

This has required renaming the existing right-to-left stylesheets so
that they can follow a naming convention derived from the base
stylesheet name.
right-to-left CSS tweaks for html_attachment view
Set right-to-left for attachment edit view

This sets the CSS direction of the title and body fields based on the
attachment's locale

As part of this, I've restructured the `AdminAttachmentsForm` JS module
to follow the same pattern as `adminEditionsForm`.
Merge pull request #1308 from alphagov/translate_html_attachments

Translate html attachments

Enable html attachments to be translated. This includes right-to-left support for those locales.

More detail: https://www.pivotaltracker.com/story/show/66142248