Welcome page: Improve implied first steps

Created by: JoKeyser

I think the current welcome page is suboptimal in that it encourages the user to immediately go to Apps. To me, this has two issues:

1. It's likely that there's a lengthy upgrade process ahead, potentially including restart of the webserver, so Plinth might be temporarily unavailable. That's at least the case for the current vagrant image; it takes a few minutes until all upgrades are installed. Users should get at least a heads-up that this is happening.
2. Many apps won't work without a working domain setup; so this should be mentioned before installing Apps.

I think both can be fixed by changing the wording, and adding a link to the domain configure page. For reference, here the current page: