Reducing size of JS bundle

[gmaclennan]

Related to #133 which is focussed on the size of the built app installer.

This issue is focussed on the JS bundle used in the electron renderer. The size of the JavaScript that is loaded in the front-end affects the initial load and parse time as well as the size of the app.

Currently the renderer JS code is not bundled - it is required at runtime from node_modules. Changing the Webpack config to bundle all files results in a bundle that is 11.17Mb. This is already an improvement on unbundled code. The main size hogs are:

7.34Mb  iD Editor
1.50Mb  @react-pdf + Yoga layout
761kb   mapbox-gl-js
744kb   @fortawesome svg icons
342kb   material-ui
264kb   Application code

The biggest size hog is iD Editor. A lot of that is probably multi-lingual support for presets and background map layer info + all the OSM icon sets.

There are numerous smaller modules like iconv-lite and different versions of core-js that take up about 150kb each that could also be trimmed if needed.