show session-key on application startup

Maybe useful to see that it really changes if unset. Also reduced key
size to 16 bytes form /dev/urandom (cosmetic reason, still enough).

isso/__init__.py

@@ -155,6 +155,10 @@ def make_app(conf=None, threading=True, multiprocessing=False, uwsgi=False):
 
     isso = App(conf)
 
+    # show session-key (to see that it changes randomely if unset)
+    logger.info("session-key = %s", isso.conf.get("general", "session-key"))
+
+    # check HTTP server connection
     for host in conf.getiter("general", "host"):
         with http.curl('HEAD', host, '/', 5) as resp:
             if resp is not None:

isso/core.py

@@ -106,7 +106,7 @@ class Config:
     default = [
         "[general]",
         "name = ",
-        "dbpath = /tmp/isso.db", "session-key = %r" % binascii.b2a_hex(os.urandom(24)),
+        "dbpath = /tmp/isso.db", "session-key = %s" % binascii.b2a_hex(os.urandom(16)),
         "host = http://localhost:8080/", "max-age = 15m",
         "notify = ",
         "[moderation]",