User menu: customusermenuitems don't map Font Awesome icons for non pix/t folders

Hi!

We added custom user menu items (customusermenuitems) and for some of them we selected a pix icon from another pix folder than the pix/t folder (we used icons from pix/i).

This is the code:

####

grades,grades|/grade/report/mygrades.php|grades

badges,badges|/badges/mybadges.php|award

####

privatefiles,moodle|/user/files.php|../i/files

calendar,calendar|/calendar/view.php|../i/calendar

messages,message|/message/index.php|message

####

language,moodle|/user/language.php|../i/publish

preferences,moodle|/user/preferences.php|preferences

The result:

All icons that use another pix folder show the fallback svg/png icon and don't use the Font Awesome representation defined in the /lib/classes/output/icon_system_fontawesome.php mapping.
For example the i/files mapping: https://github.com/moodle/moodle/blob/master/lib/classes/output/icon_system_fontawesome.php#L221

The function that processes the setting customusermenuitems should use the function that maps pix icons to Font Awesome icons.

Best, Kathrin