Re-introduce 'send email to users' feature

A merge of CE to EE nuked the 'Send email to users' button from the admin area. It used to be next to the Sort dropdown and 'New user' button in /app/views/admin/users/index.html.haml.

@rspeicher brought up the point that the location was really confusing. We have user filters on this page so it leaves the user to question whether 'Send email to users' will send to all users, or just the ones in the filtered view. The answer is neither. The feature lets you choose a group to send the email to on the next page.

We should re-introduce this button somewhere, but where? Also, when we re-add the button we also need to update the placeholder on the /admin/email page because it suggests you can send to a project or group, but only groups work in the dropdown.