Use label of vocabularies.ttl classification as headline on front page

[osma]

Currently the front page is hardcoded to show "Available vocabularies and ontologies", which may or may not be appropriate for a particular installation. For example, the Skosmos demo installation currently doesn't include any ontologies (at least described as such by the original publisher), so the headline is misleading.

Instead we could show the label of the SKOS classification embedded in vocabularies.ttl and used to categorize the vocabularies on the front page. Currently the label is set to "Skosmos Vocabulary Categories" in vocabularies.ttl.dist and not displayed anywhere.

Of course, the label in the appropriate (UI) language should be used.