Ataki/gridfs

@stvstnfrd to review commit
Someone coming across this line of code in the future will presumably
not know what "the GridFS large upload problem" is. It may make sense to
strike that phrase and combine the surrounding phrases to something
like:
> Since client-size code will batch upload large files in smaller
chunks, we also validate the filesize on the front-end (see
cms/static/js/views/assets.js).