FEATURE: Create upload_references table (#16146)

This table holds associations between uploads and other models. This can be used to prevent removing uploads that are still in use.

* DEV: Create upload_references
* DEV: Use UploadReference instead of PostUpload
* DEV: Use UploadReference for SiteSetting
* DEV: Use UploadReference for Badge
* DEV: Use UploadReference for Category
* DEV: Use UploadReference for CustomEmoji
* DEV: Use UploadReference for Group
* DEV: Use UploadReference for ThemeField
* DEV: Use UploadReference for ThemeSetting
* DEV: Use UploadReference for User
* DEV: Use UploadReference for UserAvatar
* DEV: Use UploadReference for UserExport
* DEV: Use UploadReference for UserProfile
* DEV: Add method to extract uploads from raw text
* DEV: Use UploadReference for Draft
* DEV: Use UploadReference for ReviewableQueuedPost
* DEV: Use UploadReference for UserProfile's bio_raw
* DEV: Do not copy user uploads to upload references
* DEV: Copy post uploads again after deploy
* DEV: Use created_at and updated_at from uploads table
* FIX: Check if upload site setting is empty
* DEV: Copy user uploads to upload references
* DEV: Make upload extraction less strict

db/post_migrate/20220309132720_copy_post_uploads_to_upload_references_for_sync.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class CopyPostUploadsToUploadReferencesForSync < ActiveRecord::Migration[6.1]
+  def up
+    # Migrates any post uploads that might have been created between the first
+    # migration and when the deploy process finished.
+    execute <<~SQL
+      INSERT INTO upload_references(upload_id, target_type, target_id, created_at, updated_at)
+      SELECT upload_id, 'Post', post_id, NOW(), NOW()
+      FROM post_uploads
+      ON CONFLICT DO NOTHING
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end