Allow Logging level to be setup in Admin UI
    
Currently, the logging level can only be setup only in settings_local.py which makes it difficult to change the value. Provide a dropdown in the Admin->Logging Screens that shows the various logging levels.
    
Added default value for logging_level in siteconfig.py as well as mapped the field shown in the UI to the internal variable in siteconfiguration tables.

This patch goes along with the Reviewboard change at #3852 (http://reviews.reviewboard.org/r/3852/)

Verified in fresh setup (devserver) where it defaults to the DEBUG level.  Since this only gets stored in siteconfig_siteconfiguration table, shouldn't need any migration changes. Changed the level manually in the UI and verified that DEBUG messages dont appear anymore.

Also the Filter in Admin->Server Log shows only the LogLevel selected and anything above it. For e.g. if the logging is set to ERROR, the filters in Server Log only show Error, Critical and ALL.