Make past-month stats count from yesterday
Small clean up
Add the configuration of the LDAP login attribute

 - makes the LDAP login attribute configurable (through the LDAP authentication system config in indico.conf)
 - the login attribute now defaults to 'uid' instead of 'cn' if not specified in the configuration
 - fixes #1593
Default to uid in the example config
Properly import Config
Cleanup imports
Fix blocking check; mark all CandBars as ConfBars
Consider flexibility when marking conflict bars
Improve single booking warning with only conflicts

Display a better message when during a simple booking, all the occurences are conflicting.
Also prevent from "confirming" this invalid booking and go to the next step.
Avoid querying in a loop in blocking list