move common code into server, update tests and makefile (#2425)

* move common code into server, update tests and makefile

remove backend directory, refactor

update smoke tests

server/requirements.txt

@@ -0,0 +1,22 @@
+anndata>=0.7.6 # we need to_memory(), added in 0.7.6
+boto3>=1.12.18
+click>=7.1.2
+Flask>=1.0.2
+Flask-Compress>=1.4.0
+Flask-Cors>=3.0.9  # CVE-2020-25032
+Flask-RESTful>=0.3.6
+flask-server-timing>=0.1.2
+flask-talisman>=0.7.0
+flatbuffers>=1.11.0,<2.0.0 # cellxgene is not compatible with 2.0.0. Requires migration
+flatten-dict>=0.2.0
+fsspec>=0.4.4,<0.8.0
+gunicorn>=20.0.4
+h5py>=3.0.0
+numba>=0.51.2
+numpy>=1.17.5
+packaging>=20.0
+pandas>=1.0,!=1.1  # pandas 1.1 breaks tests, https://github.com/pandas-dev/pandas/issues/35446
+PyYAML>=5.4  # CVE-2020-14343
+scipy>=1.4
+requests>=2.22.0
+s3fs==0.4.2