addressed issue https://github.com/globaleaks/GlobaLeaks/issues/828
minor fix in order to address https://github.com/globaleaks/GlobaLeaks/issues/828
Catch exceptions more strictly.

Make sure that the key file name does not exist already.
Replace PyCrypto with cryptography.

Implement some functions for handling random generation using os.urandom
minor fix against zipfile infos
removed safe umask concept
implemented uniform_answers_delay in function interacting with session_id from authenticated users
completed time constant check in XSRF comparison
minor fixes and UT (apparently) fixed