expedite checking for updates

[stalsma]

Under the version nr in the sidebar, add a link to the github release page (https://github.com/assetgrid/assetgridapp/releases), and/or the docker registry page (https://registry.hub.docker.com/r/assetgrid/assetgrid/tags) so we can quickly compare the running version to the latest version.

Even better would be a notification (An update is available. Click here to see release notes.)

Not sure how this would work for installations not using the docker container.