pending invites should be cleared when signingkey changes

[TripleSpeeder]

Scenario:
Bob invites Alice to a group.
Before accepting the invite, Alice performs account recovery.
-> Alice now has a new signingkey
-> Alice can no longer decrypt the group info from the invite

Proposed solution:
When a user gets a new signingkey, all pending invites for that user should be removed, as they can not be decrypted anymore.
This means in the above scenario, Bob needs to create a new invite for Alice. Acceptable IMO.