Document legacy Content Packs support

Upgrading a 2.5.1 setup to 3.0.0-rc.1, I could see that my old uploaded Content Packs were migrated to the new system. Just before the upgrade, I exported all Graylog entities into a Content Pack and I could also upload it once I upgraded to 3.0.0-rc.1.

Then I head to the Marketplace and tried to install a Content Pack: https://marketplace.graylog.org/addons/572ff823-62cc-468e-af17-300bcfd9932d

That failed because the Content Pack is very old and has a `null` ID on it (which throws an exception while uploading). Browsing through the Marketplace I could find quite a few of old Content Packs with the same issue.

I guess the solution to that would be to clean up the marketplace, but I'm unsure how realistic that is. Supporting old Content Packs didn't have that much priority during development, so in the end it seems to work but we didn't make totally sure we cover all cases.

As a short-term solution to make clearer which Content Packs work on 3.0 or not, we could mention in the Content Packs page that we recommend uploading and installing Content Packs created in Graylog 3.0. Older Content Packs may work, but it's not 100% sure.

We should also facilitate users to know which Content Packs are created for 3.0 or older versions, ideally also adapting the Marketplace to help with that.