* separate backend base url from frontend
This is needed for auth, and to support a different location for the backend api server,
than the frontend.
part of chanzuckerberg/cellxgene#1778
new server config parameters: app__api_base_url, app__web_base_url
Also changed api_base_url in the oauth config section to "oauth_api_base_url" to
be less confusing with the app's api_base_url
Other minor changes:
changed how the jwt decode options are handled.
Previously they needed to be set in a test case, and there was some extra logic to handle that.
Now they are handled through comfig parameters, which makes it more general.
Also, add a feature to set the CORS support credentials, which seems
to be necessary for the backend/frontend separation, at least when run
locally. This part is sort of experimental, and may be removed or changed later.
bmccandless