This commit adds a database migration to set is_recently_active=False
for channels based on the last time a message was sent.
The migration follows the same logic used in the
`update_channel_recently_active_status` management command.
This change is particularly useful for self-hosted servers, as it
avoids the need to wait a week for the
`update_channel_recently_active_status` management command to run
via cron.
