MDL-40457 Question Bank: Allow plugins to add columns to question bank view

Allows plugins to add columns to the question bank view by extending core_question\bank\column_base
Columns to display are set in $CFG->questionbankcolumns. Columns are namespaced and autoloaded to support this.
Merge branch 'MDL-40457-question_bank_plugins_columns' of github.com:MorrisR2/moodle into MDL-40457-question_bank_plugins_columns

Conflicts:
	question/classes/bank/checkbox_column.php
	question/classes/bank/question_name_column.php