Erp5 officejs fix cache

BUG: the hash to refresh the application cache is based on the ETag header of the precache manifest script response. This header is missing in the officejs server because the precache manifest is no longer a server script but a text file with the response content. In consequence, the ETag value is always null and the hash for the cache doesn't change.

To fix it, the content of the precache manifest must change to refresh the hash. A modification_date key is added to the script json dict response.