NEW: Allow configuration of initial insertion width for images and media

Moved default insertion dimensions logic from JS to PHP to allow setting through config API
Merge pull request #2146 from jonom/Configurable-default-width-for-Media

NEW: Allow configuration of initial insertion width for images and media
FIX UploadField: IE10 single click for upload trigger

Fixes https://github.com/silverstripe/silverstripe-cms/issues/644.
See https://github.com/blueimp/jQuery-File-Upload/commit/d45deb15f4934f7a2bce03040c879aca8eaa9c8a
Using extendedCan for can*
FIX: $Layout should not be executed if inside cached block

If you have $Layout inside of some <% cached %> tag, Layout should not be
executed if the cached content is available.  Prior to this commit Layout was
executed in advance of rendering the template and passed to the underlay as
a variable.  It is now passed as a closure so it is only executed as needed.

See https://groups.google.com/d/msg/silverstripe-dev/itYU50HoH50/aR6grcPIBSYJ