Add the `cellxgene schema apply` and `cellxgene schema validate` subcommands.
The first takes an h5ad file and a yaml with config information and produces a new h5ad that follows the cellxgene data integration schema.
The second takes an h5ad and checks if it follows the schema version written into its metadata.
Both are currently marked as "experimental" as the primary intended users are still at CZI.
* split out config
* add tests for base and app config, refactor client config out of app config
* refactor default config retrieval
* create config test class and helper functions
* move default_config into server to fix import issue
