do not allow enter key in dojo combobox

https://bugs.internet2.edu/jira/browse/GRP-1440

2) After input of a user's id, hitting enter instead of tab has inconsistent behavior depending on prior actions.
initially, it will change to the display name, but then flash an error "Select an entity from the search results"
continuing from above, if another id is entered, the result will be successful, but it will be the previous ID that is added to the group!
continuing from the previous entry, hitting return after a third entry will flash the error "Select an entity from the search results"
Suggested fix: remove carriage return functionality from the add member form