Split out Credential context

[neonbunny]

Currently a credential which is obtained from multiple different sources will result in multiple rows in the Credential table. This clutters the Credential view and distorts the statistics. Adjust the data model to create a 1:M structure of Credential to CredentialContext.

CredentialContext should encompass "purpose, source, source time"

The UI should surface the purpose from the first CredentialContext for its Credential table

Search should be modified to search across all CredentialContexts and Credentials.