Clean up src directory

[timonla]

While re-writing the development guidelines I stumbled upon a few things:

• Shouldn't there be a top level index.ts file for consistency? Right now it's kind of mixed.
• Shouldn't the top level function be in a sub directory as well? It does not really belong on the same level as the util directory.
• Should the util background script be included in the webpack entries? Is this not covered by the imports?

[timonla]

There should not be any index.ts files at all. We either have content.ts or background.ts scripts.