[Task] should have a prioritized list of backend features

[OriHoch]

For migration to Clearmash we will need to modify all backend calls to use the new Elasticsearch instead of Mongo

It would help to have a list of features in the backend, prioritized by how often they are used / how important they are.

Focus should be on removing mongo (which is also handled in another issue), and also a lot of the migration code.

[TheGrandVizier]

Here is what we have so far. Please pardon the mess, t'is a work in progress. https://docs.google.com/document/d/1JAn4qcTIAvmws4Qod0Zn5TFDeA-6YLQk05pWTUZiaAo/edit?usp=sharing