Embed sourcesContent in source maps

[qnighy]

This PR changes how source maps and original sources are emitted.

• Previous: the source map contains "sources": ["application.source.js"]. public/assets/application.source.js will be copied from app/assets/application.js.
• This PR: the source map contains "sources" ["application.js"], "sourcesContent": ["<the verbatim copy of app/assets/application.js>"].
  • We no longer need to add the artificial .source extension because there's no concern about file name conflicts.

There are two main purposes of source maps: for using in browsers' developer tools, and for error reporting services, like New Relic or Sentry. For the latter purpose, we usually upload source maps directly to the service and delete them before uploading to e.g. S3. Without sourcesContent bundled in the source maps, we won't be able to provide sufficient information to the error reporting service. Moreover, we need to remove application.source.js etc. as well to avoid accidental source code leakage.

Fixes #580.

Question: should we instead provide the embed_source_in_source_maps flag and let users decide the way sources are provided?