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"
Handling cut/paste is somewhat painful, but I have it working okay. I see a potential usability problem though. To cite a specific example, how should the code handle the following paste:

    foo1@bar.com foo2@bar.co

Note that the second token ends in `co` instead of `com` but it is still, by itself, a syntactically valid email address. I think there are two different options:

1. Consume both tokens as email address. This will definitely frustrate users that meant the second address to be `foo2@bar.com` and were expecting to be able to add the final `m` manually. My sense is that this kind of thing happens a lot as users try to select text with a mouse or touchpad and miss the final letter.

2. Consume only the first token as an email address and leave the second token as "in progress" input. This means that the code will never consume a full list of pasted email addresses (it will always leave the last in the list as "in progress") so it will always frustrate users that try to paste in a list of addresses. They'll have to hit an actual key (`Enter`, comma, or space) after the paste to have the final token accepted.

I think option 2 is the least bad experience, but I'm not confident in that assessment. @nicolaasmatthijs @bp323 @simong @stuartf @SamPeck, what do you think?