Add support for nested repeating_subfields

[Joonas-M-S]

Nested repeated subfields are currently rendered correctly on the first render and the data ends up into the database. However, there are some bugs

• On UI, when trying to add a new set of data for the nested subfields, Javascript creates new fields for outer fields as well
• The data is not retrieved from the database correctly. The subfields are shown empty when modifying the fields.

This pull request tries to fix these issues.