Adapt to FDP v2 property names

[dennisvang]

renamed as follows:

• modificationTime -> updatedAt
• registrationTime -> createdAt
• lastRetrievalTime -> lastRetrievalAt

todo (maybe unrelated issues):

• check why clicking fdp link fails (see FDP v2 index GET index/entries/{uuid} fails FAIRDataPoint#643)
• check why clicking trash icon fails (see FDP v2 index DELETE index/entries/{uuid} fails FAIRDataPoint#644)

fixes #169

[dennisvang]

Using this together with FDP c59d52d2 (to be part of v2.0.0), the index list page is rendered properly, as long as the list of FDPs is empty.

However, if we register an FDP via "Trigger ping" we run into the next issue on the API side, which is:

java.lang.NullPointerException: 
    Cannot invoke "java.time.Instant.isAfter(java.time.Instant)" because "lastRetrievalAt" is null

[dennisvang]

The NullPointerException mentioned above is fixed with the latest FAIRDataTeam/FAIRDataPoint#637