Allow entering of email addresses in autosuggest

- [ ] It should be possible to add an arbitrary email address to the autosuggest component. This should only be allowed when a valid email is entered and the user hits enter. This should just generate an autosuggest item that contains the email address.
- [ ] The placeholder text should read "Enter people, groups and email addresses"
I don't think so. We don't want to always hide the results (e.g. if there are actual results from the search), we just want to hide the "No results" message. That would have to be done dynamically via JavaScript, and the plugin doesn't have a hook that we can use to determine if there are any results or not. (It has a hook for formatting results, but that hook just passed each result individually to the handler; it doesn't pass the entire list of results so there's no way to check the length of that list.)