Limit number of submissions per speakers

[johanvdw]

I know that @RichiH raised this before, but I couldn't trace it back in an issue.

Just seeing someone who submitted 17 talks...

I don't believe anyone can give good talks on so many topics.

If we don't enforce the limit, at least the CfP should mention that it is discouraged. Apart from that, if we have a way to mention alternative tracks, we could avoid some duplicate submissions (perhaps even warn when a title is inserted?).