Improve long term caching

[ChristopherChudzicki]

Users should not need to re-download assets that they have already downloaded if the assets have not changed.

Our JS and CSS bundles are currently built with webpack's [fullhash] hashing. This hash changes if any aspect of the build has changed. For example:

• if you change a CSS file, in www theme, all hashes change for all themes
• if you add a comment to a webpack file, all hashes change even though no content has changed.
• As part of only load Video and Image Gallery JS when needed #648, we probably want to bundle VideoJS and nanogallery2 separately. The hash of that chunk should unless the versions are bumped.

We probably want to be using [contenthash], not fullhash.

Acceptance Criteria:

• Webpack hashes should not change unless the chunk's content has changed.

[ChristopherChudzicki]

Part of our mass publish script is to purge the CDN cache https://github.com/mitodl/ocw-studio/blob/master/content_sync/pipelines/definitions/concourse/mass-build-sites.yml#L51 If we made the above webpack improvement, would we need to change the CDN purging also in order for users to benefit?

[ChristopherChudzicki]

I think the answer to

If we made the above webpack improvement, would we need to change the CDN purging also in order for users to benefit?
is: Yes, cache purge means users would not benefit, and we should

1. Make the change mentioned above
2. Also set Cache-control: Max-age to something very large. Currently users do not load our resources from memory cache without first checking with the servers, but since we hash the js/css files, there's no need to check the servers.