DEV: Move the post and topic custom fields into a table

This PR does some preparatory work to prepare for the front-end display
of who marked an answer as solved. It migrates the custom fields from
discourse-solved of Topic and Post to a new table, except "accepted_answer_post_id"

ref: /t/-/95318

app/lib/before_head_close.rb

@@ -40,10 +40,7 @@ class DiscourseSolved::BeforeHeadClose
       },
     }
 
-    if accepted_answer =
-         Post.find_by(
-           id: topic.custom_fields[::DiscourseSolved::ACCEPTED_ANSWER_POST_ID_CUSTOM_FIELD],
-         )
+    if accepted_answer = topic.solution&.post
       question_json["answerCount"] = 1
       question_json[:acceptedAnswer] = {
         "@type" => "Answer",

app/lib/plugin_initializers/assigned_reminder_exclude_solved.rb

@@ -18,12 +18,7 @@ module DiscourseSolved
     def apply_plugin_api
       plugin.register_modifier(:assigns_reminder_assigned_topics_query) do |query|
         next query if !SiteSetting.ignore_solved_topics_in_assigned_reminder
-        query.where.not(
-          id:
-            TopicCustomField.where(
-              name: ::DiscourseSolved::ACCEPTED_ANSWER_POST_ID_CUSTOM_FIELD,
-            ).pluck(:topic_id),
-        )
+        query.where.not(id: DiscourseSolved::Solution.pluck(:topic_id))
       end
     end
   end

app/models/discourse-solved/solution.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+module ::DiscourseSolved
+  class Solution < ActiveRecord::Base
+    belongs_to :accepter, foreign_key: :accepter_user_id, class_name: :user
+    belongs_to :post, foreign_key: :answer_post_id
+    belongs_to :topic
+    belongs_to :topic_timer, dependent: :destroy
+  end
+end