[Feature Request] Show count of puzzles in each table

[arielj]

Could be useful information in the Pending and Approved tables.

I can even imagine a ⚠️ sign next to the Approved Puzzles title when the count is below 5 (currently we only have a warning in Slack, I don't think it hurts to also make it obvious in the admin panel)

[julioalucero]

@arielj do you like it? If yes, I can create the PR.