* first cut at GET /genesets route
* update existing tests to match code changes
* more GET /genesets and initial tests
* add missing test fixture
* geneset validation accepts OTA format
* genesets route: better error handling, more tests
* lint
This splits the backend into two parts: the local backend for desktop cellxgene and the AWS backend for hosted cellxgene. The local backend is in local_server while the hosted remains in server. The general idea is to copy everything from server to local_server, pull unneeded stuff out of local_server, and keep server as-is for this PR. Not touching server means all the infra and deployment code will continue working just as it did before so we can make those changes incrementally.
