"Add study to plot" feature

[abought]

Users would like to draw comparisons to other studies, as they do with LocalZoom.

We should provide an add study to plot feature . It should enable searching, filtering, and pagination in the search window. Users should be able to select one or more studies to add to the plot.

This should intersect with the skeleton regionview feature so that users can save and share their interesting plots. (This might require further design work on what options to save, access controls etc)

[abought]

Partially added via "tabix tracks" feature. Phase 2 would be a whiteboard feature- mix and match from datasets on the site.