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/string_parser.rb

@@ -33,6 +33,14 @@ class StringParser
     scanner.scan(pattern)
   end
 
+  # Skip over `pattern`, returning true if it was skipped or false if it wasn't.
+  #
+  # @param pattern [Regexp, String] The pattern to match.
+  # @return [Boolean] True if the pattern was skipped, false otherwise.
+  def skip(pattern)
+    scanner.scan(pattern) != nil
+  end
+
   # Try to match `pattern`, returning the string if it matched or raising an Error if it didn't.
   #
   # @param pattern [Regexp, String] The pattern to match.