[#6670] Send post-import email

- Sends email to all project admins after import completes (successfully)
- Doesn't send email for repo imports since they already send an email

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6670] Refactor to keep import task DRY

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6670] Fix tests

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>