This is a first step in moving the build from browserify to webpack.
The goal is to use webpack for vendor-specific builds as well,
replacing current combination of browserify, jq-based json manipulation
and inlined shell scripting.

This changes:
- time required by `yarn build` from ~100s to ~30s
- size of window.ipfs content script from 1.1M to 869K
- size of entire add-on/dist from 6.5M to 5.6M

This fixes noisy console logs on CI by switching to safe reported
when in CI env

- removed custom options that we don't need to use for now
- removed workarounds that are no longer needed

- shared webpack config as a base for all targets
- separates build targets for background page, GUI and content scripts
- bundled webext polyfill for chrome, removed two calls to `tabs.executeScript`
- detects and rebuilds ipfs proxy content script on changes (`yarn watch`)
- removes browserify libs and other leftovers

This is a PR on top of #498 

- `add-on/dist/` size decreased to `5.5M`
- shared webpack config as a base for all targets
- separates build targets for background page, GUI and content scripts
- bundled webext polyfill for chrome, removed two calls to `tabs.executeScript`
- detects and rebuilds ipfs proxy content script on changes (`yarn watch`)
- removes browserify libs and other leftovers

This will make logs non-interactive and more detailed