Allow autograde timeout to be configured

[damianhxy]

There is currently a hard limit on autograde_timeout of 899s, and there is no way to get around it short of modifying the code.

It would be good to make this limit configurable, as a class might need a longer timeout for certain assessments (e.g. 122's lightsout lab). Alternatively, we might modify the logic to allow an admin to set the value to be greater than 899s.

(But this is "not a big deal" for 122)

Note: if the autograder times out, Autolab simply interprets the last line as the score. The onus is on the instructor to enforce their own timeout if needed. However, it might be worth considering handling timeouts specially, to avoid the setting of erroneous scores.