Make dates respect current locale (deprecations!)

And make comments use it in a smart way (it's not but it works)

Deprecate "extra.date_format" and "extra.comments.locale" and replace it
with "date_format" and "date_locale" in translation files

content/demo/comments.ar.md

@@ -0,0 +1,8 @@
++++
+template = "article.html"
+title = "ActivityPub/​Fediverse comments demo (العربية)"
+[extra.comments]
+host = "toot.community"
+user = "sungsphinx"
+id = "111784580759701190"
++++