focus next input when you select flickr image result tile

fixes: #CNVS-26048

test plan:

when you click / hit "enter" on an image result in 
the flicker search results, it should focus the next 
input  box in the form (the "alt" field), so you don't 
have to tab through the other 500 search results.

this makes things nicer for keyboard-only users.

Change-Id: I453141146a4097bff7e981ac39ac2921a5876530
Reviewed-on: https://gerrit.instructure.com/69245
Tested-by: Jenkins
Reviewed-by: Simon Williams <simon@instructure.com>
Reviewed-by: Aaron Cannon <acannon@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>

app/coffeescripts/views/tinymce/InsertUpdateImageView.coffee

@@ -124,6 +124,7 @@ define [
       @setSelectedImage
         src: $a.attr('data-fullsize')
         alt: $a.attr('title')
+      @$("[name='image[alt]']").focus()
 
     onFileLinkDblclick: (event) =>
       # click event is handled on the first click