Cleanup: Buildversions are numbers, so default to zero
Partial pick: Drop dependency on Djblets’ SiteSettingsForm

We can do way with it by subclassing Django’s Form directly.

Pick notes: Partial pick of khaledhosny/pootle@1eb6c2bd. The rest of the
    changes don't apply since we dropped the ability to alter the settings
    from the home page in b4d46d65.

Conflicts:
	pootle/apps/pootle_app/views/admin/adminpages.py
	pootle/apps/pootle_app/views/index/index.py
Replace djblets: Reduce dependency on djblets

Also this code accomplishes the same thing and is much more easy to
understand than djblets one we were using.
Expose djblets usage: Get site title and description

Exposing each separate use for siteconfig makes easier to replace it.

Add functions to retrieve site title and description. This avoids us
retrieving all the site config in the place where we need the title or
the description.
Expose djblets usage: Save build versions

Exposing each separate use for siteconfig makes easier to replace it.

This moves the ability to save buildversion to pootle_misc. This avoids
us retrieving all the site config in the place where we save the build
versions.
Expose djblets usage: Retrieve build versions

Exposing each separate use for siteconfig makes easier to replace it.

This moves the code to retrieve buildversion to pootle_misc. This avoids
us retrieving the build versions in the place where we need the build
versions.
Replace djblets: Add a new PootleConfig model to save the build versions
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.
Pootle Site: Return default value if no PootleSite object exists

This fixes the tests failures.