Observation - show flag if pictures exist

As an API user, I want to be able to filter observations that have (or have not) pictures (ImageRefs)

AC

• field hasImages (bool) is added to ObservationAggregate. This field is aggregated from underlying ImageRefs - if any ImageRef exist it is set to true (otherwise false)
• it is possible to search by this field, aggregate_hasImages_is: true
• view_observation_aggregates is changed and SQL is added to next release file in folder sql/releases(DROP existing view_observation_aggregates and create new one)
• Existing tests are running

Review

• filed hasImages (bool) is added to ObservationAggregate. This field is aggregated from underlying ImageRefs - if any ImageRef exist it is set to true (otherwise false)
• it is possible to search by this field, aggregate_hasImages_is: true
• view_observation_aggregates is changed and SQL is added to next release file in folder sql/releases(DROP existing view_observation_aggregates and create new one)
• Existing tests are running

Question: ask Lars if it would be better to have count of images (for sorting by number of images)

^ hasImage suffices