Drop the ability to write e.g. `create alias foo -> char:bar` in a BUR
to change the tag's type as a side effect. You can only use these
tag type prefixes in tag edits now.
This feature was only intended to be used in tag edits. The fact it
worked elsewhere was unintended behavior.
This feature was problematic because it relied on `Tag.find_or_create_by_name`
automagically changing the tag's category when the tag name contained a
tag category prefix, e.g. `char:hatsune_miku`. This meant that merely
looking up a tag could have the side effect of changing its category.
It was also bad because `find_or_create_by_name` had a hidden dependency
on the current user, which may not be set or available in all contexts.
015c6dc7db