Core: Add multi-user/multi-instance support

[beaulebens]

At the core level, the importers should ideally support multiple users. Everything would work very similarly, but you should have the ability to create multiple instances of each importer, and connect them to different Keyring tokens (users), then configure them differently.

This will require changing how options are stored/handled, and will also require some UI thought around how to allow for the creation of new instances, and how to go between existing instances (list them out, link to each, etc)