"Allow late submissions" setting

[jgat]

Add allowances for users to submit a problem without academic penalty. This is implemented as a flag set in the user's submission data. Also added functionality into the client and web interfaces to display info about this flag.

Note: this feature is not fully implemented. For the time being, there's no need to merge this pull request, but if you do, it won't break the existing functionality (I think; I haven't tested it very extensively). Checkboxes below indicate progress:

• [x] Store "allow late" flags on the server (including support functions to set the flag and check the flag)
• [x] Modify server handlers to respond with a new response type indicating "submitted late without penalty"
• [x] Modify client to accept the new response type and display to user correctly
• [x] Modify /cgi-bin/progress.py to report the new status
• [ ] Add handlers to the server to allow admins to set the late flag
• [ ] Add functionality to the admin client to use those handlers (see #46)

Non-core goals:

• [ ] Add support for unsetting the "allow late" flag, including:
  • [ ] Server support functions to appropriately modify/retrieve the data from storage
  • [ ] Server request handlers
• [ ] Consider a different storage method on the server (e.g. integrating with the submission_log file)

[jgat]

Due to the new web-interface plans for the admin interface, the scope of unfinished work relating to this branch has changed, so I'm merging this as "complete".