Image and Description output merging

We need to merge in all length description fields from equivalents on output. This is done in the same way as it is done for title and standard description fields: using a function to find the first non-null field in the list of euqivalence-precedent-ordered equivalents.

Additionally we need to merge images from equivalents on People, which are not Content but are Described (as is Content) so image merging is moved up to work on Described not just Content.