Allow jinja blocks to have default values
Allow jinja blocks to have default values

This removes the last instance of manually constructing jinja
tags from the processor, which helps us avoid typos in individual
keys.

robinwhittleton/govuk_template,froddd/govuk_template,jonheslop/govuk_template,jonheslop/govuk_template,quis/govuk_template,alphagov/govuk_template,robinwhittleton/govuk_template,jonheslop/govuk_template,alphagov/govuk_template,quis/govuk_template,froddd/govuk_template,alphagov/govuk_template,quis/govuk_template,quis/govuk_template,jonheslop/govuk_template,robinwhittleton/govuk_template,alphagov/govuk_template,jonheslop/govuk_template,robinwhittleton/govuk_template,alphagov/govuk_template,froddd/govuk_template,froddd/govuk_template,froddd/govuk_template,quis/govuk_template,robinwhittleton/govuk_template