Acknowledged contact alerts should be cleared about app-ios

[scottleibrand]

May be fixed by the same change as #115

[ivnsch]

If matched only new keys (delta from last fetch), this problem would be solved, but we'd miss possible new CENs for old keys. Not sure whether there's a solution for this or perhaps it's admissible, given that users that report symptoms are likely to take protective measures / distance.

The other solution would be doing the filtering after the matches. When the alerts are ACK-ed we'd only set a "deleted" flag on them (instead of deleting), and if we get the same matches they'd not overwrite these entries, so they'd stay deleted. There would be a regular job deleting too old alerts (more than 3 or 4 weeks), which we need also to delete old CENs.

[scottleibrand]

New TCNs for old keys should be disallowed: the rak needs to be rotated whenever we submit a report, so that no one receiving the report can impersonate the reporter by generating new TCNs with the newly-revealed rvk and tcks.

[ivnsch]

For 0.2 I'm marking the alerts as deleted (as described). Can you open a new issue for the 0.4 backend?

[ivnsch]

Alerts stay cleared now.