archwiki: LocalSettings.php: give registered users the 'writeapi' right

Adding/removing pages from the whitelist when using the Vector skin requires the write API now.

Without this, regular users get this error:

You're not allowed to edit this wiki through the API.

A user reported this issue in #archlinux-wiki.

/cc @archlinux/teams/wiki/admins

mentioned in commit 6c7dbb13

merged