Don't treat *_(cosplay) tags as being invisibly aliased when a character tag is aliased. For example,
if toosaka_rin is aliased to tohsaka_rin, and toosaka_rin_(cosplay) is later added to a post, don't
magically translate the tag to tohsaka_rin_(cosplay). Instead, treat it as an error to create a
*_(cosplay) tag for an aliased character tag.
This removes some of the complexity and magic behavior surrounding *_(cosplay) tags.
Factor out most of the tag edit logic from the Post class to a new
PostEdit class. The PostEdit class contains the logic for parsing tags
and metatags from the tag edit string, and for determining which tags
were added or removed by the edit.
Fixes various bugs caused by not calculating the set of added or removed
tags correctly, for example when tag category prefixes were used (e.g.
`copy:touhou`) or when the same tag was added and removed in the same
edit (e.g. `touhou -touhou`).
Fixes#5123: Tag categorization prefixes bypass deprecation check
Fixes#5126: Negating a deprecated tag will still cause the warning to show
Fixes#3477: Remove tag validator triggering on tag category changes
Fixes#4848: newpool: metatag doesn't parse correctly
