autocomplete: clean up handlers for itemSelectEvent

The handlers that execute when an autocompleted item is selected are really
specific and cannot be factored out. Nevertheless, some cleanup:

- rationalize indentation
- remove redundant copy/pasted comments
- remove redundant group handling in MentionsAutoComplete
- remove redundant checks on 'oData.nname != undefined', used to
differentiate user vs. group completion, in places that only allow user
autocompletion