Fixes login/logout may not work (viewer.js cache)

The `cache-control` headers set in [entry.py](https://github.com/tonio/c2cgeoportal/blob/66e60dac07bf7ac6008c93be11ad22a9f0ff4f15/c2cgeoportal/views/entry.py#L74) is imho too agressive for this route.
`viewer.js` shouldn’t  be cached.