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"
Since the input element is an `<input>` and not a `<textarea>`, I think the browser will swallow any newlines in pasted text and insert spaces instead. So what we'll see from the above is

    foo1@bar1.com, foo2@bar2.com foo3@bar3.com foo4@bar4.com

To extract the email addresses from that list we'd have to treat the space character like a comma. I think that's okay, so long as no user or group has a display name that validates as an email address.