Provide an "unmaintained" message for abandoned translations

@hsbt @postmodern @chikamichi We thought about something like this some time ago.

This patch adds an include for unmaintained languages (and applies it to bg, ko, pt, tr).
A languages home page can be marked as "unmaintained" by adding

    {% include unmaintained.html %}

in the source file.

This produces a message on the generated homepage (in red color):

```
This translation of www.ruby-lang.org is unmaintained
and might contain obsolete information or even errors.
Please refer to the <a href="/en/">original English site</a>
for the most up-to-date content.
```