FEATURE: Promote bookmarks with reminders to core functionality (#9369)

The main thrust of this PR is to take all the conditional checks based on the `enable_bookmarks_with_reminders` away and only keep the code from the `true` path, making bookmarks with reminders the core bookmarks feature. There is also a migration to create `Bookmark` records out of `PostAction` bookmarks for a site.

### Summary

* Remove logic based on whether enable_bookmarks_with_reminders is true. This site setting is now obsolete, the old bookmark functionality is being removed. Retain the setting and set the value to `true` in a migration.
* Use the code from the rake task to create a database migration that creates bookmarks from post actions.
* Change the bookmark report to read from the new table.
* Get rid of old endpoints for bookmarks
* Link to the new bookmarks list from the user summary page

spec/serializers/post_serializer_spec.rb

@@ -254,11 +254,7 @@ describe PostSerializer do
     context "when a Bookmark record exists for the user on the post" do
       let!(:bookmark) { Fabricate(:bookmark_next_business_day_reminder, user: current_user, post: post) }
 
-      context "when the site setting for bookmarks with reminders is enabled" do
-        before do
-          SiteSetting.enable_bookmarks_with_reminders = true
-        end
-
+      context "bookmarks with reminders" do
         it "returns true" do
           expect(serialized.as_json[:bookmarked_with_reminder]).to eq(true)
         end
@@ -275,16 +271,6 @@ describe PostSerializer do
           end
         end
       end
-
-      context "when the site setting for bookmarks with reminders is disabled" do
-        it "does not return the bookmarked_with_reminder attribute" do
-          expect(serialized.as_json.key?(:bookmarked_with_reminder)).to eq(false)
-        end
-
-        it "does not return the bookmark_reminder_at attribute" do
-          expect(serialized.as_json.key?(:bookmark_reminder_at)).to eq(false)
-        end
-      end
     end
   end