display an error message on ajax error
hook handle_ajax_error to DELETE too
Replace alert with flash message
make annotations readOnly if user is not logged in

Closes #38
disable delete button in readonly mode
disable selection in readonly mode
enable readonly mode for images too
add an icon for each filetype

Closes #45
audio annotations: return a 401 instead of a 500
audio annotations can't be moved in readonly mode
don't make audio annotations editable in readonly mode
audio: disable delete button in readonly mode
disable creation of new audio annotations in readony mode
Merge branch 'guest-mode'