gitlab-ci: job to check for merge conflicts with Weblate

Now that @linsui is starting to merge translations in from Weblate, I added these two jobs to hopefully catch future breakage before it hits the buildserver, e.g. fdroidserver#1143

@proletarius101 I couldn't make sense of the complicated rules: sections fdroiddata now has. Could you advise what rules: these new jobs should have?