userpage: use form langfile, move login strings to common

login-related stuff was moved into common using the langmover script, so
that the user page doesn't have to use the admin language files.

lang/form/sl-si.json

@@ -57,4 +57,4 @@
             "plural": "Potrebnih je vsaj {n} posebnih znakov"
         }
     }
-}
+}