Correct logic and behavior of FigureMedia, external IIIF images in TOC grids

[cbutcosk]

This PR corrects behaviors of the FigureMedia class to resolve #1232 and for overall correctness:

• Refactors annotations and sequences computed properties to store factory-created annotations in a private property. Previously the factory for these properties was unnecessarily being called multiple times to create instances, upon each usage of the properties.
• Eliminate unexpected behaviors for annoations by moving the execution of annotation asset handling to after the figure's core image assets have been processed. Annotation instances immediately produce JSON for their IIIF representation so they require asset paths populated by the image asset handling method.
• Renames asset handling methods for clarity.
• Refactors TOC grid item image component to use pre-calculated derivative media paths for applicable images, adds a test ensuring that behavior.