Allow selecting default posting language instead of auto-detect (#7828)

* Allow selecting default posting language instead of auto-detect

* Enable default language setting in credentials API

* Fix form saving

app/javascript/styles/mastodon/forms.scss

@@ -140,6 +140,8 @@ code {
   }
 
   .input.with_block_label {
+    padding-top: 15px;
+
     & > label {
       font-family: inherit;
       font-size: 16px;