Define sort ordering for completion conditions

At the moment, completion conditions are being displayed in the following order:

• View
• Receive a grade
• Custom completion conditions

For activities with no custom completion conditions, this is fine, as "view" will always be achieved before "receive a grade". However there are some cases where an activity with custom completion with have some/all of those conditions met before receiving a grade. To make it clearer to the user, it would make sense to order the conditions in the UI so that they are (as best as possible) listed in the order they would usually be completed in. For example, in the assignment activity:

• View
• Submit assignment
• Receive a grade

would make more sense than:

• View
• Receive a grade
• Submit assignment

Therefore we should require custom completion conditions to define their completion sort order, including both custom conditions AND the core completion conditions of "View" and "Receive a grade".