Allow users to bookmark variants and genes

[holtgrewe]

Is your feature request related to a problem? Please describe.
Bookmarking variants, would be a useful feature.

Describe the solution you'd like
Logged-in users should be able to bookmark variants on the server, connected to their account. Display a "bookmark" button that allows users to bookmark sequence and structural variants as well as genes.

• add models, schemas, API in the backend
• add API, store code on the frontend
• integrate with the view code

The model should store something like this:

interface bookmark {
    user: string // uuid
    type: "seqvar" | "strucvar" | "gene"
    obj_identifier: string  // genomic variant / HGNC ID
}

All of the above with tests.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.