Refactor developer convenience scripts (#1377)

* Cleanup the backend-dev convenience method

* Add the frontend_dev convenience method

frontend_dev is a soup-to-nuts convenience method for setting up the FE
development environment with node running a the client code on port 3000
with the a separate cellxgene package serving the API over port 5005 in
the background.

The script can be run from Finder.

* Update the developer scripts documentation

* Remove the 'test' make target in the client Makefile

Rationale:
* Given how long the smoke tests take to run, it is unlikely that
  developers will want to run all tests together.
* It is unlikely that developers will have set up the backend server
  properly for the tests to pass.
* Available commands should be safe-ish and not lend themselves to
  confusing errors.
* You can still group tests by concatenating them in a make command, as
  in `make unit-test smoke-test`.
* This target isn't used in any of our CI pipelines -- KISS.

* Some version of python3...

* Minor typos in docs

* Respond to feedback from @bkmartinjr

* Make adjustments so that DATASET path is predictable

* Simplify environment defaults a bit

environment.default

@@ -2,3 +2,5 @@
 CXG_SERVER_PORT=5005
 CXG_CLIENT_PORT=3000
 JEST_ENV=dev
+DATASET="$(git rev-parse --show-toplevel)/example-dataset/pbmc3k.h5ad"
+CXG_OPTIONS=--debug