Replace djblets: Add a new PootleConfig model to save the build versions
Replace djblets: Add PootleSite model to store the title and description
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
Drop unused djblets code
Do not use djblets middleware

Also drop code that is unused now that the middleware is not used.
Replace djblets: When upgrading migrate to PootleConfig ASAP

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