Email sending feature for sustaining members

[Xpirix]

For #29

Cc @andreasneumann

Feature summary

• Add a field for "Tech Email" under the sustaining member's form
• Add the checkbox "Receive News and Crowdfunding Information" under the sustaining member's form

• Add the "Emails" button on the homepage and the sidebar
• Paginated list of previous emails
• Compose email page with the following fields: Subject, body, sponsor levels (multiple), email type (tech, admin or both), cc (Can be used by the sender to receive a copy of the email)
• Email details page
• Allow resending email: can be used to resend a failing one, or resend one that has already been sent.
• Allow deleting email: this will do a soft delete (hide from the list but doesn't delete the record from the DB)

email-sending-feature.mov

Note:

• I think it's better if we make the recipients Bcc so the email addresses will be kept private.
• Only Members that has the "Receive News and Crowdfunding Information" option checked will receive the email, if no member has set this option, the email will be saved as a draft and can always be resent later.
• Should we also allow sending attachments alongside the email?