Disable problem after accepted submission

[Jl5her]

Summary

After a team has successfully solved a problem. Disable future submissions.

Basic example

Motivation

Although why would a team make another submission, stop them from accidently doing it.

[Jl5her]

There are two places this can be implemented.

1. Disabling the button on the front end in blue/submit.html
2. Submit method in contest.py
   I suggest doing both, just in case someone has the problem page cached before the team submitted the problem successfully and their submit button is not disabled (thus allowing them to submit again).