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

scripts/frontend_dev

@@ -0,0 +1,42 @@
+#!/usr/bin/osascript
+
+set DATASET to system attribute "DATASET"
+set CXG_OPTIONS to system attribute "CXG_OPTIONS"
+
+on gotoProjectRootDir()
+  tell application "Finder"
+    set current_path to container of (path to me) as alias
+  end tell
+  tell application "Terminal"
+    do script "cd " & (POSIX path of current_path) & ".." in front window
+  end tell
+end gotoProjectRootDir
+
+on newTab()
+  tell application "System Events" to keystroke "t" using command down
+  delay 0.05
+end newTab
+
+tell application "Terminal"
+  activate
+  reopen
+
+  my newTab()
+  my gotoProjectRootDir()
+  if DATASET is not equal to "" then
+    do script "export DATASET=" & DATASET in front window
+  end if
+  if CXG_OPTIONS is not equal to "" then
+    do script "export CXG_OPTIONS=" & CXG_OPTIONS in front window
+  end if
+  do script "./scripts/backend_dev" in front window
+
+  my newTab()
+  my gotoProjectRootDir()
+  do script "sleep 1
+    # This tab is for the client hot-reloading node server. It will wait until cellxgene is built before continuing. Please hang in there while the other tab completes.
+    while [ `ps aux | grep dev_setup | grep -v grep | awk '{ print $2 }'` ]; do sleep 1; done
+    cd client/
+    make start-frontend" in window 1
+
+end tell