Sending emails to members

[madprime]

General Checkups

• [x] Have you checked that there isn't already an existing issue that describes what you report below?
• [x] Have you checked that there isn't already an open pull requests for this issue/update/change?

Description

Open Humans staff/admins would like the ability to send emails to members via the site directly, for newsletters and other items.

Currently this is available in the bulk_email management command, but this has a lot of issues - e.g. it requires locally running code with production keys, and retains no record of what was sent. It can take hours for an expert admin (i.e. one of our few developers) to set up, review, and run an email.

Ideally we'd have a feature built in to the site, available to staff/admin, that's more like the project messaging feature available to projects.