Review Extending All Dates

[cycomachead]

This will be a meta issue to fix everything about setting/extending due dates.

• ensure no dates disappear when sending an extension request
• option to extend late due date or not be a delta
• send a late due date if present
• Store assignment release dates?

[cycomachead]

I think this is mostly addressed by the recent extend late due dates.

[cycomachead]

See #213