Switch manifest file extension to webmanifest

Created by: stramel

Based on the spec, the manifest should have a .webmanifest extension.

It is also recommended by, webhint.

Resources supporting this:

• https://www.w3.org/TR/appmanifest/#media-type-registration
• https://developer.mozilla.org/en-US/docs/Web/Manifest
• https://github.com/webhintio/hint/tree/master/packages/hint-manifest-file-extension