* 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
* Small fix for handling display versions
Making a distinction between __version__ and the version we display in the info panel (displayr_version).
The hosted cellxgene can overwrite the display_version using a plugin.
Improve version handling in the customized assets
* Add server plugin system
Plugins are optional modules loaded at runtime. Specification:
* Plugins are loaded from the server.plugins module (directory
server/plugins)
* The import_plugins method is run as part of the initialization of the
server module in __init__.py
* Add plugins to the EB build process
* Remove bit of dead code
* Respond to feedback from @bmccandless
