make embargo template theme-able

Use "Stanford-style" theming to enable you to put your own embargo notice up.

@flowerhack: this is a tiny change to enable us so put up our own themed embargo message.  We are pulling in the same change to our own instance and deploying tomorrow (see PR #2833).  Indeed you can see what our themed version looks like in the screenshot attached to that PR.

Note that you probably think that this is a pretty ugly way to do theming and overrides.  It is, sorry about that.  When we did our original pass at theming edX, nearly a year ago now (!) we weren't able to get a cleaner way working.  You'd think that you could just use template search precedence so that one template would override the others, but it's complicated by edX using two templating systems, and templates including each other.  This should get cleaned up.  In the meantime, this change is consistent with how we do it elsewhere.

Thanks.