Move viewer image caption to the top on hover

Addresses #717
Tweak styles for "popup" image caption in viewer

Give ``<figure>`` relative positioning, so absolutely positioned
``<figcaption>`` is at top of the figure and not its parent.

Add margin so caption follows padding around image.

Addresses #717
Always show caption at bottom of asset and popup

Instead of moving the caption element to the top of the container,
copy its contents to a separate element and always show the caption
at the bottom.

This prevents content jumping around when hovering toggles the
popup. It also makes the caption immediately visible as the user
scrolls below the top of the image.

This approach no longer allows for a pure CSS solution as we need
to use JavaScript to create a separate element.

Cleanup trailing commas in object literal definition.

Addresses #717
Add rgba test to Modernizr build

This is needed to provide classes to trigger a fallback for
semi-opaque backgrounds for popup captions.

Addresses #717
Use transparent PNG for background when no rgba support

For IE8 and other browsers that don't support rgba, use a transparent
PNG for the background of the popup caption.

Addresses #717
Merge branch '717-viewer_popup_captions' into develop
Update changelog for #717
Add favicons to builder and viewer templates

This is something that's been bugging me for a long time.

Factor favicon-related markup in base.html into favicons.html so
it can be included in templates that don't inherit from base.html.
Remove data links from popup caption

Also refactor caption styles

Addresses #717
Make sure <figcaption> element has text

Make sure an asset's ``<figcaption>`` element has text before
copying it for a popup caption. Some assets may have ``<figcaption>``
elements containing things like a list of datasets, or attribution,
but no caption text.

Addresses #717
CSS classes for asset_id and asset type

Include CSS classes for the asset_id and the asset type in the
asset's container element.  This will facilitate type-specific
styling. It also makes it easier for developers to look up an
asset when viewing front-end display of information.
Hide popup captions for YouTube videos

The popup captions look weird over YouTube videos, especially
since the video embed is fixed-width.  Hide them using a CSS
style matching the ``asset-provider-youtube`` class.

Addresses #717
Don't send site contact notifications from user address

Instead, send them from ``settings.DEFAULT_FROM_EMAIL``.

Sending from the user's email address was causing the message to
get rejected by the GMail SMTP server when the user's address was
a GMail or Hotmail (and perhaps others) address because the sending
SMTP server wasn't GMail's or Hotmail's.

Closes #886.
Update tests for #886 changes
Remove circular imports from Haystack signal processor

See http://stackoverflow.com/a/17364366/386210

Addresses #887
s/ugettext/ugettext_lazy/