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.
    
Added a new field called logging_level in Admin->Logging that allows the user to select a log level. Grouped the field under the general "Logging Settings".

This patch goes along with the Djblets change at #3851 (http://reviews.reviewboard.org/r/3851/)

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.