Add installation instructions for Debian
Use subprocess.DEVNULL when possible

Also, this change causes to open os.devnull at most once, thus fixes #353
Fix notifications after dataset creation
Round of pylint
Update authors file
Move clone dataset operation to db class
Tool to remove submissions
Be less pessimistic about PHP memory-hungriness

Before we used just 1M ints to cross the 128MB threshold, this turns
out not to be enough in some systems (read: mine).