fixed styling of input fields in cas
 suppress form submission on enter in cas-cell
 bug in form validation

git-svn-id: https://dev.geogebra.org/internal/moodle/moodle-assignsubmission_geogebra/branches/MOODLE_27_STABLE@3204 872ce066-4876-4fca-a6fc-23b94bf576d1