utils: Add function to generate pseudo-random strings.
cookies: Add a second csrf cookie.

Based on code provided by @fuz-woo.

For some reason, it doesn't work for me when I use Python 2, but it does
when I use Python 3 and needs further investigation, because this alienates
a huge fraction of the users.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>