Unified properties in response schemas + minor fixes

[gvasold]

Responses to the listing endpoints (/factoids, /statements etc.) looked like this:

{
   'protocol': {..},
   'factoids': []
}
{
   'protocol': {..},
   'statements': []
}

When writing tests I got repeatedly confused with the different names. So I unified them for all listing endpoints to

{
   'protocol': {..},
   'data': []
}

Also made some minor changes:

• re-introduced sortBy for factoids because someone might want to sort by @id createdBy etc.
• fixed the response schema for /persons, /statements and /sources. These schemas were already defined, but not used in the paths defintions.