FEATURE: Track last_viewed_at datetime for channel members (#22294)

Whenever a user opens a channel or marks it read, we now
update the last_viewed_at datetime for that channel membership
record. This is so we will be able to show thread unread indicators
in the channel sidebar that clear independently of the main thread
unread indicators. This unread functionality will follow in another
PR.

plugins/chat/db/migrate/20230627044755_add_last_viewed_at_to_user_chat_channel_memberships.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class AddLastViewedAtToUserChatChannelMemberships < ActiveRecord::Migration[7.0]
+  def change
+    add_column :user_chat_channel_memberships,
+               :last_viewed_at,
+               :datetime,
+               null: false,
+               default: -> { "CURRENT_TIMESTAMP" }
+  end
+end