Build: switch from gulp to webpack (via npm scripts)

[infokiller]

This will improve development because:

• Using imports will make dev tools (such as editors and linters) aware of file dependencies. Right now we have a few of eslint-disable comments for issues related to this, and autocomplete doesn't work well for me in vim.
• Some files in the src directory are actually dependencies that we deploy there for loading the extension from this directory. Mixing source files of this project with others is not good, for example web-ext lint reports errors on files from dependencies.

Of course, we need to make sure that there are source maps and debugging using devtools is just as easy as it is now.

This should be pretty easy, example extension: https://github.com/mdn/webextensions-examples/tree/master/webpack-modules

Related: #75