Clarify messages in extension achievement items when reduced scoring is enabled

[dlglin]

The messages for extension items only talked about the due date, which would be confusing to students, as they would probably associate "due date" with the full-credit deadline. This clarifies what will happen to both the reduced credit date and the close date, trying to use similar terminology to the messages at the top of the set.

[dlglin]

This fixes #2905

[pstaabp]

Is there a way to test this without achieving Level 3? Overall, the code looks fine, but wanted to make sure that it works.

[somiaj]

You can create an achievement that gives you as many items as you want for just attempting it once.

In the ModelCourse there is an extensions.axp (allows importing of the achievement) and extensions.at (actual achievement) files that you can use as a template. Though by default it won't give you all possible extensions, you'll have to add all the other extension items you want to test.

$globalData->{ExtendDueDate}      += 10; 
$globalData->{SuperExtendDueDate} += 5; 
$globalData->{ResurrectHW}        += 3; 
return 1;

If you have already earned this achievement, you can reset / unassign / reassign it to earn it again. You can also adjust the numbers to have any number of items to test.