#38 - Use HTTP Headers from published datasets URL to retrieve versioning of the application

[jrmartin]

Use HTTP Headers from published datasets URL to retrieve versioning of the application.

The SDS Viewer relies on published datasets published on this server, which is a large file. The application relies on downloading this file first to retrieve all the IDs of the published Datasets. Since it's a big file and it doesn't get updated every day, it's not necessary to download the file each time the application loads, so it's only downloaded whenever there are changes to the file.

In order to figure out if there are changes to the file, this PR does:

1. Uses the above URL to retrieve its headers
2. Uses the headers to determine the last date it was modified.
3. Stores in browser's local storage a cookie with the Version Date and Datasets IDs.
4. On each load of the application, the local storage version is compared to the one coming from the server's headers, if different then the Datasets IDs on local storage are updated as well as the version (date last modified).

This functionality existed prior to this PR, but it was manual versioning. This PR makes it so instead of versioning being manual, it uses the headers's last modified date as the versioning number.