userlists: add list 3xmigrate

This list is intended as a easy and bullet proof way of triggering optional migration.

This also fixed script that generates pot files. It swaps what file is merged to which. This is correct order as otherwise new strings are marked as old ones in output.

cznic/userlists/files/definitions.json

 {
+	"3xmigrate": {
+		"title": "Migration to Turris OS 5.x (HIGHLY EXPERIMENTAL)",
+		"description": "This is experimental migration to the latest version of Turris OS. Make sure that you thoroughly read https://docs.turris.cz/geek/tos3_migration/ before enabling this!",
+		"visible": true
+	},
 	"api-token": {
 		"title": "Access tokens",
 		"description": "A Foris plugin allowing to manage remote API access tokens (for example for use in Spectator or Android application).",