Tidy up client JS dependencies (#146)

Runtime JS dependencies are obtained as follows:

- dependencies for the main client window (eg jquery, ace) are obtained
  via cdnjs.cloudflare.com, and so we don't need anything beyond than
  the script tags in index.html.
- dependencies needed inside the iframe (eg react, for gists which use
  react-based libraries like purescript-react or purescript-thermite)
  are obtained via the new 'shims' mechanism introduced by #128 / #136.

The files I am removing in this commit do not appear to be in use. I've
also removed non-purescript dependencies from bower.json, as they also
do not appear to be in use.

Author: hdgarrood

client/bower.json

@@ -17,11 +17,6 @@
     "purescript-web-html": "^2.3.0"
   },
   "devDependencies": {
-    "ace": "^1.2.8",
-    "jquery": "^3.2.1",
-    "underscore": "^1.8.3",
-    "react": "^16.1.0",
-    "mathbox": "^0.0.5",
     "purescript-arrays": "^5.1.0",
     "purescript-bifunctors": "^4.0.0",
     "purescript-console": "^4.1.0",