don't allow tags of different cases
don't allow tags of different cases

Previously they were allowed to be added, but all searches and
operations treated them as case-sensitive, creating an inconsistency.
The new implementation will change new tags with different case than
existing ones to the version currently in the database.

This may cause some slowdown in collections with a very large number of
tags since the only way to do this at the moment is to scan through
every one of them. Changing the format tags are stored in in the future
may be useful.

weihautin/anki,holycrepe/anki,florianjacob/anki,xuewenfei/anki,Arthaey/anki,subfusc/anki,LucasCabello/anki,abeyer/anki,jkitching/anki,jakesyl/ruby-card,hssm/anki,socialpercon/anki-1,go38/anki,sunclx/anki,weihautin/anki,hssm/anki,Stvad/anki,kenjhim/anki,jakesyl/ruby-card,eduOS/anki,ospalh/libanki3