Cellxgene-gateway

* Fixed bug 45 affecting multi-level S3 folders
* Added extra_scripts to cache_status page

* Added missing __init__.py

* Added support for itemsource interface, allowing s3 hosting
* Removed support for http file uploads
* Only set wsgi.url_scheme when EXTERNAL_PROTOCOL is set (see issue 43)
* Dropped flake8 due to conflicts with black
* Added code coverage metrics

* Added support for ProxyFix

* Fixed bug with annotations (missing annotation.js asset)

* Minor fixes to enable cellxgene 0.16.0
* Added CELLXGENE_ARGS to enable passing additional arguments to cellxgene
* added metadata/ip_address endpoint