Image admin component

Generalized base admin image upload tool in Site that other packages can subclass.

Matches what was recently added for `SiteAttachment`.

Requires https://github.com/silverorange/admin/pull/24 for the Image delete tools, https://github.com/silverorange/admin/pull/25 for the memcache clearing to work correctly when a new image is uploaded, and https://github.com/silverorange/admin/pull/27 for replacing the existing dataobject when editing.