Email integrity check

Implement integrity check of email signatures and certificate chains

created branch 46-email-integrity-check

created branch 46-email-integrity-check-2

mentioned in merge request !55 (closed)

mentioned in merge request !56 (merged)

added needs review label and removed DevInProgress label

added ReadyForQA reviewed labels and removed needs review label

added ReadyForMergeInMaster testedByQA labels and removed ReadyForQA label

Tested the endpoint is working.

mentioned in commit ec688929