[web] Use a specific manifest.json file for development

[dgdavid]

Problem

We've changed the content-security-policy in #419 via manifest file for allowing HMR. However, that change must not reach a production environment.

Solution

Added a manifest.dev.json and keep manifest.json as it was. Depending on the NODE_ENV value the resulting div/manifest.json will contain the proper settings.

Testing

Tested manually by running npm run build and NODE_ENV=production npm run build and checking the content of the dist/manifest.json

[coveralls]

Coverage: 77.784%. Remained the same when pulling 91c8052 on fix-manifest into ad0580d on master.