Instead of being configurable, made xhr_paging always enabled.

This option made no sense to be turned off because it makes for a better
user experience to prevent a full page reload when paging.
Cleaned up preview windows after we've tested their content.
Now requiring only the jquery-ui JS that we use.
Merge pull request #2733 from refinery/enable-xhr-paging-by-default

Instead of being configurable, made xhr_paging always enabled.
Rectify changelog conflict.
Merge pull request #2736 from refinery/preview-functionality-cleanup

Preview functionality cleanup
Always set config.dragonfly_secret
Updated changelog with pull request references.
No need to specify xhr_paging, since 622cce9cabbf875221dde00b0e8902d653974df5
Merge pull request #2737 from anitagraham/ImageDragonflySecret

Set config.dragonfly_secret
Vertical admin tabs

Having admin tabs at the top running horizontally means they have to fit into a finite space. If there are too many, they go into a carousel, which is very inconvenient for navigation, and makes the rightmost tabs invisible to users.

I've put the admin tabs vertically at the left of the admin area. This gives as much vertical space (with scrolling) as needed.

- positioned admin tabs vertically at left
- wrapped admin content in #admin_container div
- corresponding #login_container div for login page
- more rounded corner mixins
- box-sizing mixin
- admin_background mixing provides faux column gradient background for admin area (modern browsers and IE10 up)
- SASS variables for $menu_width and $standard_rounding
- sortable_menu.js setting changed to y axis
- removed jcarousel and images

Conflicts:
	core/app/assets/images/refinery/carousel-left.png
	core/app/assets/images/refinery/carousel-right.png
	core/app/assets/stylesheets/refinery/layout.css.scss
- replaced arrow_switch.png with arrow_up_down.png and arrow_left_right.png
- moved SASS utility mixins to a separate file
- generalised admin_background() mixin to faux_columns_2()

Conflicts:
	core/app/assets/stylesheets/refinery/layout.css.scss
- removed different padding/margin for active tab
- Increased admin width.
- Removed admin rounding.
- Gave menu a defined background.
- Changed width of search box.