Source Cleanup/Validation

[acoffman]

Add a database and model constraint to ensure a source must have a citation id.

Remove the erroneous source 2841, flag or fix EIDs impacted and add test cases to ensure evidence cannot be submitted without a source.

[susannasiebert]

Some of the affected EIDs are rejected so we might just need to remove the source from them altogether

Up-to-date list can be found here: https://civicdb.org/sources/2841/summary

[susannasiebert]

Per comments on the remaining EIDs for this source (https://civicdb.org/events/genes/6281/summary/variants/2752/summary/evidence/7438 and https://civicdb.org/events/genes/11314/summary/variants/2672/summary/evidence/7229) can be totally deleted and then we can delete this source.

[susannasiebert]

2841 and its associated EIDs (and their subscriptions, notifications, and events) has been removed.