refs https://github.com/TryGhost/Team/issues/1175
We found the ETag header sent when serving the Admin template for /ghost/ was not changing between versions which after an upgrade could result in out of date cached content being served containing links to JS/CSS files that no longer existed.
The culprit is weak etags served by Node's `send` package, coupled with Admin template filesize not changing between versions and `npm pack` setting a fixed modification date for every file. See https://github.com/pillarjs/send/issues/176 for more details.
- updated the Admin app's controller to read the template and generate an md5 hash of the contents so we can serve a strong ETag header value when serving the `/ghost/` html
