Tags are case sensitive

Tag filter allows to add case sensitive tags when creating new article or translation. When one of existing variants of the same tag added to the article, all other variants are also added there automatically.

Steps:

1. Create an article with tag "java"
2. Try to create another article with tag Java

AR: Article is created; also tag "java" is added automatically (see screenshot in attach)
ER: Impossible to create tags with different case

note, that right now there is several tags with different case in production (so we need to update it and delete duplicates)