flags/appeals: replace is_resolved flag with statuses.

Replace references to the `is_resolved` field with the `status` field.
Post flags were marked as resolved when a post was approved (but not
when the post was deleted because it went unapproved). The status field
supercedes the resolved field.

test/factories/post_flag.rb

@@ -3,6 +3,5 @@ FactoryBot.define do
     creator
     post { build(:post, is_flagged: true) }
     reason {"xxx"}
-    is_resolved {false}
   end
 end