Export - Component defined fields (e.g. Student) must be exposed in contact-type drop-down for Selected field export

Currently, there is no way to export Student fields when using the "Select fields for export" mode. Needed fixes are:

1. If a component (e.g. Quest in this case) has defined a class and fields for export, this class should be added to the "contact type" drop-down in the hierarchical select.

2. ALL Fields with <export>true</export> in the component class XML file (in this case Quest/Student.xml) should be available in the "select fields" drop-down (after selecting contact type = Student).

NOTE: Quest will potentially want to export any of the quest_student table fields - so I have updated Student.xml to set all the fields for export = true.

When testing this - please make sure that fields where option_value ID's are stored have the option_value.title in the export cells. Also make sure that column headers use the "friendly titles" - e.g. Total Household Income, not household_income_total.

(We will be removing all non-core/location fields from the "Primary" export mode - I'm entering this a separate issue. This means that the few Student fields currently being output for Primary exports will no longer be included in that mode.)