Add the ability to show statistics over the currently available dataset

[bhermann]

Users of the management platform should be able to see how much data is currently present in the elasticsearch database.

Possible user questions might include:

• How many unique identifiers (i.e. projects) do we currently have in the system?
• How many of these unique identifiers have been successfully processed with which tool?
• How is the evolution of this data over time? (e.g. via charts)