Flextensions overwrites manually input extensions

[pancakereport]

Current logic checks for extensions already requested by a student via flextensions and preserves the longest requested extension via flextensions. This logic does not include a comparison of a students current due date (which may have been manually input by an instructor) with the newly requested extension via flextensions. If the Gradescope API allows, logic should be updated to maintain the longest of either the extension requested via flextensions and the current due date.