Fixed Issue #723,This is a partial fix

This is a partial fix where mutiple assets drop is rejected.
Yet to put the + ikon as indicator for drop or some reject symbol for multiple files.
Fixed Issue #723,added fix for file navigcation in case of readonly notebooks

Fixed the issue reported by Arko that due to fix on #731(text drag between cells restored) the asset drag check for historical notebooks got broke.
Fixed Issue #723,added fix for file navigcation in case of readonly notebooks

This is a partial fix where mutiple assets drop is rejected.
Yet to put the + ikon as indicator for drop or some reject symbol for multiple files.
clarify -enter key combos; punct
add RCS doc link
Fixed Issue #723,Added reject symbol for multiple file drag and drop

Added reject symbol for multiple file drag and drop
Fixed Issue #723,

added reject symbol for readonly files as well.
ignore rcloud.client build file
move drop effect to event handler on scratchpad wrapper

so that (+) is only shown when over that area
make drop text clearly singular

until multiple file upload is implemented
always show file upload pane when uploading asset

& minor improvements to upload results posting
Merge branch 'pr/744' into develop