Link gene sets to annotation terms

[sidneymbell]

From @aopisco:

If a user creates a cell type annotation as in #996, allow them to link each label to a specified gene set as evidence (with congruent, ontology-based names for labels and gene sets).

See also #1353

[aopisco]

This could generalize as a property of the annotations, e.g. right now colors are associated to each category, we could think about associate marker genes at each cell label

[brianraymor]

Per PM triage, moving to Product Backlog for prioritization.