Chapter leaders can email the entire chapter

[lilliealbert]

(Needs to be done after #442)

Add a feature to the chapter show page that allows chapter leaders to email everyone in the chapter.

It should be similar to the event "Email Attendees" feature, with a Subject & Body (but without a way to specify who in the chapter the email should go to).

The emails should only go to users that haven't opted out of chapter announcements (to be implemented in #442).

The email should also include a link to the settings page to opt out of chapter announcements, similar to event announcement emails, something like: "If you'd like to opt out of future chapter announcements, you can do so on your settings page." <-- with a link

[kariabancroft]

I'd like to work on this one