Collect all env vars in one, easy-to-find place (#1149)

* Collect all env vars in one, easy-to-find place

Past state:
* Default environement variables were stored in both client/package.json
and client/__tests__/e2e/config.js
* Constants that should have been linked--like the cellxgene server port
during testing--were repeated.

With this commit:
* All environment variables are parameterized
* All environment variables are packaged in default env files

* Move npm scripts to client Makefile

* Respond to feedback from @seve and @bkmartinjr

Makefile

@@ -1,3 +1,5 @@
+include common.mk
+
 BUILDDIR := build
 CLIENTBUILD := $(BUILDDIR)/client
 SERVERBUILD := $(BUILDDIR)/server
@@ -5,7 +7,6 @@ CLEANFILES :=  $(BUILDDIR)/ client/build build dist cellxgene.egg-info
 
 PART ?= patch
 
-
 # CLEANING
 .PHONY: clean
 clean: clean-lite clean-server clean-client
@@ -72,9 +73,11 @@ smoke-test:
 .PHOHY: fmt
 fmt: fmt-client fmt-py
 
+.PHONY: fmt-client
 fmt-client:
 	cd client && $(MAKE) fmt
 
+.PHONY: fmt
 fmt-py:
 	black .