Store 'registered by' when registering a data collector

[karolikl]

We need to store who a data collector was registered by.

[cathinkaw]

After discussions with @aninek and @RFMoore we have discovered that they are rather interested in "who is responsible for this volunteer". That is the data verifier, and in most cases the data verifier is the one registering the data collector.

The thougth is therefore that upon registration of a data collector the data verifier needs to be provided, and if it's possible to get the "person id" from the logged in user, this will be set as the default data verifier.