Allow entering text into a textarea in the submission page

[prandla]

I was discussing all the different contest systems at BOI and came up with a few feature requests that we could copy from other contest systems :)

Someone mentioned that being able to copy code from the text editor and paste it into the judge system is really convenient. I think we've also considered adding this in the estonian fork. We could add a textarea to the task submission page as an alternative to the file select box.