Allow users to sort submissions (#22)

[SuperCl4sh]

Fixes #22.

[SuperCl4sh]

Due to the nature of select2.js, users are able to enter multiple sort options, which is entirely useless since the probability of having a tie to break is near zero. In the future, perhaps it would be worth resolving this minor issue.

[zecookiez]

Looks promising, but the sort option seems to be broken on the Best Submissions tab:

Sorting seems to be going in descending order on the All submissions tab, not sure if that's intended?

[SuperCl4sh]

I believe it appeared to be sorted that way due to the Best Solution tab overriding the sorting options with runtime ascending. I have changed this and added ascending and descending sorting options for memory and runtime. In the future, perhaps a sorting option for submission source size could be added.