This was implemented by Matt Petro of the University of Wisconsin - Madison Engineering
School and Math Department. Many thanks. Reviewed by and committed by Tim Hunt.
This adds a new Overrides tab to the UI, with sub-tabs Group overrides and User overrides.
Each of those lists all the overrides that currently exist, and lets you manage them and
create more.
When a quiz is being attempted, the override that applies to the current user is combined
with the current quiz settings loaded from the quiz table (normally called $quiz).
If there are both user and group overrides, then just the specific user override is used (more specific).
If the user is in several groups, then the overrides are combined to give the most permissive set of options.
There is one new database table quiz_overrides, to store the overrides.
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