Allow 4 sets at wdvenn, use venn.js for chart

[Vladimir Alexiev]

For http://tools.wmflabs.org/wikidata-todo/wdvenn.html

• allow up to 4 sets
• IMHO the best graphing software is https://github.com/benfred/venn.js/ since it allows ANY number of sets (most others are limited to 3).
  • Example of use: http://vladimiralexiev.github.io/CH-names/README.html#sec-3-2 for an
  • Try it here: http://vladimiralexiev.github.io/CH-names/cranach-venn.html