Add info about verification status

[SReichelt]

Add overlay icons to library items to distinguish

• fully proved theorems (green),
• theorems that lack a proof (yellow),
• theorems that have one or more proofs but none of them are fully verified from first principles (no icon, but the user should be able to find out where the gaps are),
• definitions or theorem statements where inline or well-definedness proofs are missing (yellow, but at some point we will no longer accept such submissions),
• definitions or theorem statements where inline proofs depend on unproved results (also yellow?), and
• definitions where everything is OK (no icon).

When submitting a proof, at least the icon of that item should be updated immediately. In general, the verification status could be determined during a check of the entire library, see #18. It should probably be stored in the _index.slate files; if the library check touches those, that's OK.

Possibly include more info about dependencies, like e.g. in Metamath.