[IMP] web_editor: user text copy/paste

Users copy/past text from various king of document into the editor.
These texts contain useless line breaks that should be removed before
pasting. Also, the editor currently add a paragraph element between
each line.

To correct this behaviour the onPaste callback of the summernote lib
is used. However for some reason, this is triggered after the text
is pasted (we should update and refactor the lib in a better way).
So this commit uses a hack: undo the past thanks to the undo command
then paste the formatted text we want.