Make tags case-independent

[glennlunder]

MIgrated from issue #158 on the trac-server.

Menace: "What the topic says; the current system apparently leads to situations like this, where the latter is spelled with a capital K in the tag itself."

http://demozoo.org/productions/tagged/4klang/
http://demozoo.org/productions/tagged/4klang_1/

Gasman: "Makes sense - ticketed. On the subject of tags - for a project at work I used the jquery library Tag-It http://aehlke.github.io/tag-it/ (not to be confused with the Django library Taggit, which Demozoo is already using :-) ) which provides a more intuitive input field, has support for autocomplete suggestions, deals nicely with the minefield of different ways of entering them (comma-separated, space-separated, quoting for multiple words) and turned out to be really easy to integrate. We should definitely use it, and solve a lot of our tag-entry problems in one go... "

[glennlunder]

http://demozoo.org/forums/post/547/#post-547

[gasman]

Fixed in 157c631 - all tags are now slugified (converted to lower-case and spaces replaced by dashes) so these duplicates don't arise.