This change adds new email templates for project invites, one for
existing users, one for new users. It changes the project invite code
to use the new template for existing users.
Issue: https://github.com/storj/storj/issues/5860
Change-Id: Ic7b14a677277ea6c25ee527d03f709474fc05f83
Emails should not contain user input that could be used by malicious
agents to deliver a message. Usernames have been removed from
account activation emails, and project names have been removed from
project invitation emails.
References storj-private#133
Change-Id: Ic05921149b409145df109c0966ea5dfd86d86eb1