UI updates for builder asset/datasets (#767).

(Attempting the git rebase technique ... makes it kind of ginormous.)

In builder asset and dataset forms, mutually-exclusive options are 
grouped under tabby pills. File inputs get thumbnails. Various other
UI refinements—"Edit Image" titles and such.

- Add MutexGroupedInputForm subclass of Backbone.Form
- Models.Asset specifies mutexGroups
- fields in mutexGroups are marked as class "mutex-group"
- MutexGroupedInputForm.render re-jiggles DOM elements
- Add "action" template variable to builder asset create/edit view.
  Title for view is "Create [Asset Type]" or "Edit [Asset Type]".
- Simplify help labels for asset edit fields.
- Use fieldset tag to group tabbed options in MutexGroupedInputForm.
- Add a mutexGroupName option to MutexGroupedInputForm. This shows
  up as a legend labelling the tabbed fieldset.
- Use MutexGroupedInputForms for asset dataset creation.
- Update CSS so that tabbed groups are a little more distinguished.
- Allow for different help for asset views in different states.
- Add help items for image editing, creation, and display.
- Add help item for asset type selection.
- Change verb for creating a new asset: "Add", rather than "Create".
- Make asset creation form validation error list styled with sitewide
  error list styling.
- Don't ignore Backbone.Forms built-in validator errors.
- Add client-side validation string for case of zero values
  provided (assets only).
- Add punctuation (colon, period) to multiple-value validation error
  string.
- Add FontAwesome warning icon to error messages.
- Provide error coloring for GroupedMutex option nav.
- Don't use the "required" input attribute.
  Browser support and UI for in-browser validation is inconsistent.
  For now, we'll just roll our own.
- Render file input thumbnails in builder.
  File inputs for datasets and images (of any sort) have thumbnails.
  Un-set inputs show a [?] icon, while inputs with a set file show
  either a "set" file icon or the image thumbnail itself.