Improve workflow for reviewing curation results

[nawatts]

Frequently, projects have a second round of curation where results from multiple curators are reconciled and finalized. However, since the result database records are tied to assignments and assignments must be unique for a curator/variant within a project, there is no way for a curator to go back and add a "final" result without overwriting their original result.

A workaround is to copy variants and results into a new project and modify the results there. However, this requires duplicating variants and associated records (annotations, tags, samples).

Thus, we should add the concept of multiple "rounds" of curation into the application. A project would have one or more rounds and a curator could be assigned the same variant in each round. Instead of assignment records being unique for a project/curator/variant, they would be unique for project/curator/variant/round. Results for each round could also be downloaded separately. The overall project results would contain results grouped by curator/variant, with only the result for the most recent round in which that curator/variant was assigned.