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
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Merge pull request #1651 from pferreir/v1.2

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