Cleanup: Remove unnecessary checks
Don't migrate buildversion on updatedb to PootleConfig

It won't work because the necessary tables are not yet present.
Also it is not necessary because this upgrade is already performed later
in the upgrade management command.
Recover ability to save buildversion using djblets

Since updatedb cannot rely on having the buildversions in PootleConfig
(see previous commit), we need to get back the legacy code to save the
buildversion using djblets.
Bugfix: Specify removed field instead of relying on model

Since we removed the description_html fields in b44dcf03 (in 2.5.1 and master)
we have to specify the fields instead of relying on the model, because they
do not longer exist there.
Styleguide: Use single quotes for template tags and filters in HTML attributes

So we avoid clashing with the HTML double quotes as much as possible.
Dev docs: Add guidelines on how to use migrations, upgrade and initdb
Bugfix: There is no flush_checks
Cleanup: Move code to upgrade management command

Since it is the only place where it is used it makes sense to be there.
This allows to keep the module __init__.py for code more focused on the
upgrade machinery internals.
Cleanups

To ease further refactoring and improve readability.
Cleanup: Provided build versions are always integers

So avoid the conversion.
Cleanup: Now `buildversion_for_fn` always returns an int

So no need to convert the retrieved build version in other places.
Cleanup: Pass the function instead of the function name
Cleanup: `filter_upgrade_functions` already receives the function build

Still wondering why this code worked.
Cleanup: Just do the comparison instead of calling function
Cleanup: Remove intermediate upgrade function
Bugfix: Use the right build versions