Check whether Lagotto version is current

[mfenner]

Many installations of Lagotto are not running with the latest version of the software. This has become a support issue. We can add a reminder to the status page if the version needs to be upgraded, and we can use the Github releases API to check for the latest version.

We should do pessimistic versioning, i.e. only show a reminder if there is a major 4.0 or minor 3.6 new version, not for a patch such as 3.6.4.