Use blocktrans where we need to substitute a value

[stefanor]

The change in #427 never actually worked, because you can't substitute variables inside {% trans %}. In fact, for a non-trivial variable like this, one has to use {% blocktrans with foo=bar %} to make a simple variable name available inside the translation string.

[coveralls]

coverage: 97.162%. remained the same when pulling 9fb83bc3d77bdfbfc83f88e55aa0de60c5cdf724 on stefanor:trans-variable into e4e5c8a9ca4f8c5fbf6e32bbe228617216c47089 on macropin:main.