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"
Once we allow commas, we should make sure that pasting lists of email addresses separated by commas or enters automatically recognises the email address

e.g.

```
foo1@bar1.com,
foo2@bar2.com
foo3@bar3.com
foo4@bar4.com
```