Add import of last question submission time from DOMjudge

[mmcinnestaylor]

The Team model currently has two attributes which are populated by contest results from DOMjudge: questions_answered and score. In the event two or more teams are tied on both of these values in DOMjudge, the tie break is determined by which team submitted their last solution earliest. Support for importing this value should be added in order to display contest results which are consistent with DOMjudge's scoreboard.

• this attribute should be added to the Team model
• similar to the score attribute, a PositiveSmallIntegerField(default=0) field should suffice for importing this value