Move needed require.context into an auto-written dot file and deprecate app.js

[KyleAMathews]

Every site needs a loadContext function to create a Webpack context (Webpack doesn't let you create require contexts with variables meaning loadContext has to have a hardcoded path which means it must have a known relationship with the pages source directory.).

Right now you need an app.js file for this function. We should make that optional by auto-writing a .gatsby.js file in the root directory of a site. This would also remove a potential roadblock from people creating a Gatsby site + allow us to easily change/add to this loadContext function.

[andreypopp]

I think it should be possible to wrap compiler.inputFileSystem into some sort of virtual filesystem and write app.js there not to touch the real filesystem.

[KyleAMathews]

That would be very cool.

This is what I found from googling around webpack/webpack#806 Looks like @theplan wasn't able to quite get it going...

Oh also this shama/webpack-stream#3

[andreypopp]

So another idea is to move all site related metadata/config into a single place (I think package.json could be that place, we can store config under gatsby key instead of a separate TOML file) which could serve as an entry point for webpack compiler.

We would need another webpack loader which would transform package.json into loadContext function.