Add a data source description as hover text

[emtwo]

This PR allows an admin to add a description for a data source which is shown as hover text.

It is the fourth part of a series of PRs for schema enhancements. It is preceded by #3293

[1] Schema viewer drawer #3291
[2] Schema admin configuration #3292
[3] Schema query samples #3293
[4] Data source descriptions #3401 (This one)

[arikfr]

Thanks, @emtwo, can you please:

1. Add screenshots to your pull requests showing the UI changes.
2. Make a list of which commits belong to which PR.

It's really hard to review without it.

[guidopetri]

@emtwo , one more :) We've updated a lot of things now that we're Community-driven so - if you're still interested in getting this merged - would you mind rebasing off master to re-run the CI, as well as updating merge conflicts?

We're trying to clean up our PR todo list, so if you're not interested, that's fine - we'll close the PR in about a week if we don't hear back. If you're interested in reopening the PR afterwards, we would also very much welcome that.

[justinclift]

This one's probably not useful as-is. It seems to require a series of other PRs to already be merged, some of which we've closed already.