Replace djblets: Add PootleSite model to store the title and description

Also adjust the GeneralSettingsForm to use PootleSite.
Replace djblets: Use a single function to get Pootle build, wherever it is
Cleanup: Save as POOTLE_BUILDVERSION instead of legacy BUILDVERSION

Since the previous commit the code works fine just using POOTLE_BUILDVERSION
instead of BUILDVERSION.

Note that this requires treating the build version as a string.
Replace djblets: Use a single function to get the Toolkit build
Replace djblets: Retrieve title and description using new PootleSite model

But keep the old functions to retrieve the data stored using djblets.
Replace djblets: Save build versions using PootleConfig only

Even when saving old build versions. This requires migrate to PootleConfig
ASAP on the upgrading cycle.

This will allow to simplify a bit our code to save and retrieve the build
versions so it only relies on the new PootleConfig.
Cleanup: Separate code to get legacy and new build versions
Drop unused djblets code
Do not use djblets middleware

Also drop code that is unused now that the middleware is not used.