Provide a Admin UI setting to change Log Levels

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/)

Testing done:
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.

Fixes bug 1859.
Reviewed at http://reviews.reviewboard.org/r/3852/