Support for full mail accounts to be linked to a Project via its Notification Scheme

Currently, a project can only be linked to an email address when setting up its notifications. Following on, JIRA still uses the email account setup as an "outgoing mail server" to actually send the emails.

This causes problems such as the ones outlined at JRA-34817.

A mismatch between the email address and the actual account to be used to send emails is treated by some public email services as an spoofing attempt and therefore, the mails continue to be sent from the address linked to the outgoing mail account rather than the one configured for the project.

Under JRA-34817 we added a warning for this situation as a short-term fix.

To completely avoid this problem, the outgoing mail functionality needs to be rewritten such that a user is able to add several "outgoing mail servers" (email accounts) and then be able to link a project to a full account rather than just an email address.