Localisation updates from http://translatewiki.net.
Fix #15704: Wrong description of writing custom_functions
Fix #15698: system warning when editing profile

When user clicks Submit button to edit an existing profile, without
first selecting one from the list, the following system warning was
triggered: 'extract() expects parameter 1 to be array, boolean given'.

The code now ensures that a profile has been selected before redirecting
to account_prof_edit_page.php, and issues a meaningful error message if
not.
Reenable ability to clear default profile

Regression introduced by fe8e3672f555063fadbf652e3c557f5a438a3313

Fixes #15698
Updated CREDITS file in preparation of 1.2.15 release
Bump version and update release notes for 1.2.15