Allow entity Import/Export

## Background
Currently, entity definitions in Menas cannot be exported. The only thing you can export is a file uploaded for a Schema entity.

## Feature
It would be useful if users could export and import any entity definition as a json.

An open question is how to handle conflicts (cases where a dataset definition with the same name and version already exists). This conflict resolution should be drawn up in a shared doc and once resolved posted here.

This task is similar in nature to #594, but meant for manual single item import/export, while #594 is meant for direct promotion via HTTP and in bulk.

## Expected task
1) API
2) UI adjustment