posts: factor out post edit logic.

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

app/logical/post_query_builder.rb

@@ -574,12 +574,6 @@ class PostQueryBuilder
       end
     end
 
-    # Parse a tag edit string into a list of strings, one per search term.
-    # @return [Array<String>] the list of terms
-    def parse_tag_edit
-      split_query
-    end
-
     class_methods do
       # Parse a simple string value into a Ruby type.
       # @param string [String] the value to parse