improve image-upload

improve image-upload

* create dropzone for editing in jot-modal
* customize css for dropzone to make it more beautiful (smaller
  margin/paddings...)
* change position of dropzone-element dircetly to textarea, so button
  for upload is small and under textarea (it's more clear)
* remove previews after upload. this shows the upload-progress, but
  preview is not needed after upload. you can't do anything with the
  preview, and dialog gets too big. You see successful upload, when
  bb-code link to image is inserted on cursor-position in textarea