Make the `trellis:` constants overwritable

[FullLifeGames]

While setting up trellis (triplestore), I encountered the fixed prefix (e.g. for data TRELLIS_DATA_PREFIX with trellis:data as prefix).

In my current use case, there is already an RDF store which we want to migrate to be used by Trellis.
However, we would not want to migrate all our named graphs since the URIs have some meaning.

Therefore, my suggestion would be to make the constants like TRELLIS_DATA_PREFIX configurable so that the user can decide which prefix they would like to use. The default value can be kept the same, being able to set it in the application.properties would be sufficient.