The mailing lists member sync was too elementary to
be used on production properly. This was for example
due to the single calls to MailChimp for each seperate
Contributor.

Now during sync, only updates are called to MailChimp, not
*all* contributors. Also, the batch operation is used instead
of seperate calls. Further, some query tweaks were used that
expect database calls to improve significant.

`vet_submitted_reports` is updated such that the amount of logic is
reduced and 1/2 view is dropped.
`submit_report` has extra precheck before submitting report, checking
if submission is open for reporting. Else, only submit report
if user has invitation.