minor HTML fixes

I was in the mood for bug squashing today so I ran adhocracy.de through the w3c validator. That did only complain about few things:

-   Thumbnail badges did not have the `alt` attribute set
-   a link used the obsolete `name` attribute
-   The html element did not have a `lang` attribute

The last one is actually really important for a11y (#492) as screen readers need to know which language they are reading. We could think about adding lang attributes to even more elements. E.g. we could set it on static pages and user generated content as the translation with them is either different from adhocracy or non existent.