Add _get_count method
* counts the amount of objects in this tag
* tries to do so without actually fetching the objects
* this lets us fetch counts for large items (ie Variations in CB4856)
** this would break before
* also lets us fetch the count before trying to populate a large table
* related to #1154, #1572
Use _get_count in Strain and Variation
* add showing counts fallback to datatable display
* #1154, #1572