Activity import: allow multivalue custom data

Before: the current activity import does not allow the import of multi value custom data.

If I have a favourite primary colours field, I could not import a CSV with the a column for this field with the value Red, Blue. It would look for the entire string Red, Blue and complain that Red, Blue is not a primary colour.

After: we can now import multi value custom data using the comma as a separator.

The proposed patch explodes any multi valued custom data fields on the , character.

Note that this does not stop people from using commas in labels of custom data fields. It just means that if people want to make use of the importer to import multi value custom data, they will need to ensure that their labels don't have any commas in them.

Saying the same thing again using some different words...

We are assuming that if people want to import multi value custom data then they will use a comma seperator to seperate fields and understand that any labels will not contain commas as a result.

This feel fairly reasonable to me.