Provide default label when configuring downloads

Problem/Motivation

When configuring downloads from view results, under 'Preconfiguration for "Generate csv from selected view results"', if the Label is left blank, the column header will be blank. It would be more useful if the field name was used.

Proposed resolution

When the Label is blank, use the value of "Field name".

Remaining tasks

Agree on what to do. Implement.

User interface changes

Label can be left blank in many cases.

API changes

None.

Data model changes

None.