Fix notifications

https://gitlab.com/soapbox-pub/soapbox/merge_requests/120/diffs#diff-content-917c7fb80cf426cfae1e7baaa21b3d90ccb2b29c

db/migrate/20191201204843_add_last_read_notification_to_user.rb

@@ -0,0 +1,5 @@
+class AddLastReadNotificationToUser < ActiveRecord::Migration[5.2]
+  def change
+    add_column :users, :last_read_notification, :bigint
+  end
+end

db/migrate/20191201205006_add_foreign_key_constraint_to_user_last_read_notification.rb

@@ -0,0 +1,5 @@
+class AddForeignKeyConstraintToUserLastReadNotification < ActiveRecord::Migration[5.2]
+  def change
+    add_foreign_key :users, :notifications, column: :last_read_notification, on_delete: :nullify, validate: false
+  end
+end

db/migrate/20191201210057_validate_add_foreign_key_constraint_to_user_last_read_notification.rb

@@ -0,0 +1,5 @@
+class ValidateAddForeignKeyConstraintToUserLastReadNotification < ActiveRecord::Migration[5.2]
+  def change
+    validate_foreign_key :users, :notifications
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_09_17_141707) do
+ActiveRecord::Schema.define(version: 2019_12_01_210057) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -823,6 +823,7 @@ ActiveRecord::Schema.define(version: 2019_09_17_141707) do
     t.string "chosen_languages", array: true
     t.bigint "created_by_application_id"
     t.boolean "approved", default: true, null: false
+    t.bigint "last_read_notification"
     t.index ["account_id"], name: "index_users_on_account_id"
     t.index ["confirmation_token"], name: "index_users_on_confirmation_token", unique: true
     t.index ["created_by_application_id"], name: "index_users_on_created_by_application_id"
@@ -934,6 +935,7 @@ ActiveRecord::Schema.define(version: 2019_09_17_141707) do
   add_foreign_key "user_invite_requests", "users", on_delete: :cascade
   add_foreign_key "users", "accounts", name: "fk_50500f500d", on_delete: :cascade
   add_foreign_key "users", "invites", on_delete: :nullify
+  add_foreign_key "users", "notifications", column: "last_read_notification", on_delete: :nullify
   add_foreign_key "users", "oauth_applications", column: "created_by_application_id", on_delete: :nullify
   add_foreign_key "web_push_subscriptions", "oauth_access_tokens", column: "access_token_id", on_delete: :cascade
   add_foreign_key "web_push_subscriptions", "users", on_delete: :cascade