staff grading xmodule - initial version

update staff grading: bigger button, prevent accidental multiple grade submits

staff grading xmodule - render with ajax, put js in local function

fix two small SGA bugs

fix multiple-grade-entry bug due to event propagation

add annotated file upload capability to SGA

SGA bugfix log_action

SGA fix ungraded

correct misspelling in SGA instructions

SGA another typo

max length on comments

staff grading xmodule needs weight for proper grade computation

fix SGA to publish grades back to module_render