[PATCH] Ability to select the user profile fields to include in gradebook exports

This commit introduces two new settings in the admin page:

• 'exportuserfields' for built-in user profile fields
• 'exportcustomfields' for custom fields

Both of these settings are string lists (comma-delimited) with the shortnames
of the fields that be included in the columns of grade exports.

This feature allows admins to hide fields they don't need (for example the
institution field in a one-school site) or add extra custom fields they are
interested in.

(the attached patch is against 1.9.3+)