Fix for gathering keys from nested fields

[jegra]

This addresses an issue found when a Flexible Content field is nested inside a Repeater. The image thumbnails are missed in this scenario, resulting in blank cards for the various Flexible Content options.

It will apply the following changes :

• a check for 'repeater' field types in the retrieve_flexible_keys_from_fields method
• a recursive call to retrieve_flexible_keys_from_fields if a 'repeater' field is encountered