MM-53747: Create job to encode older image paths (#24073)

Bifrost now encodes all image paths. Due to this
one-way translation, we need to encode all the older
image paths as well.

After this is done, we can remove the double-lookup.

https://mattermost.atlassian.net/browse/MM-53747

```release-note
NONE
```

Co-authored-by: Mattermost Build <build@mattermost.com>

server/public/model/job.go

@@ -35,6 +35,7 @@ const (
 	JobTypePostPersistentNotifications  = "post_persistent_notifications"
 	JobTypeInstallPluginNotifyAdmin     = "install_plugin_notify_admin"
 	JobTypeHostedPurchaseScreening      = "hosted_purchase_screening"
+	JobTypeS3PathMigration              = "s3_path_migration"
 
 	JobStatusPending         = "pending"
 	JobStatusInProgress      = "in_progress"

server/public/model/migration.go

@@ -42,4 +42,5 @@ const (
 	MigrationKeyAddCustomUserGroupsPermissionRestore   = "custom_groups_permission_restore"
 	MigrationKeyAddReadChannelContentPermissions       = "read_channel_content_permissions"
 	MigrationKeyElasticsearchFixChannelIndex           = "elasticsearch_fix_channel_index_migration"
+	MigrationKeyS3Path                                 = "s3_path_migration"
 )