Each module that logs content now uses a module-level logger. This adds
some context to log messages and makes it easier to scan for log
messages related to a given module. It also would allow for filtering of
logs, if the consuming application wanted.

The default log message format in djblets.log includes the logger name
for each log message.

Unit tests and pyflakes pass.
Tested various log messages to see that the names were appearing in the

log output.