Fix mess of nginx/php/mediawiki upload size limits

[hcooper]

Unpicked the complex mess of upload size limits, and documented them.

tl;dr -

• 10MB is the upload limit (already set in php.ini), mediawiki should provide friendly errors.
• both nginx configs will allow 11MB in order to not interfere in the process. It currently has contradictory settings.

This fixes the problem that mediawiki says the limit is 10M, but nginx will fail it if it's over 5M.

[BenjaminPelletier]

I thought I had confirmed an 8MB upload worked way back when, but apparently we've always been at war with Eastasia...

Thanks, this clear and I don't see anything that could be incorrect.