Remove files built by webpack

[njj]

This PR updates the .gitignore to ignore the main.js file that is built by Webpack. Typically any scripts built by module loaders or task runners are ignored, as they can be created on the fly by developers.

[vkarpov15]

IMO keeping the compiled file is a good idea so people can start the repo immediately without building anything. The behavior that I'm trying to get is "npm install and npm start are sufficient to run everything" (although admittedly I haven't added the start script yet...)

[njj]

Fair enough @vkarpov15, this just means that you will have to ensure you build this file on every change and subsequent commit.

[vkarpov15]

Yeah I'm not a huge fan of that TBH, but I also like to honor the contract that npm install + npm start is sufficient to be able to go to localhost:4000 and see a working site. What would you think about putting webpack in a postinstall or prestart script?