Improve scrolling efficiency on dashboard

[brimoor]

Infinite scrolling of images in the dashboard is awesome! So awesome that we made it a front-and-center feature that every user will definitely interact with. As a result, it's important that we optimize its performance as much as possible.

This will be a recurring issue, but here's an initial list of low-hanging fruit that we brainstormed:

Scrolling efficiency ideas

• cache samples that have already been loaded so that scrolling upwards does not require reloading
• prefetch the next page of results
• optimize image annotation rendering (whatever low-hanging fruit may exist)
• consider storing image thumbnails in the DB (this would be an internal-only field, not loaded when the user interacts with Samples) and show those in the samples view
• asynchronously load content so that content appears as fast as possible; for example, show a gray/solid-colored placeholder image while the actual image loads so that scrolling doesn't appear to "hang" until enough images load that the next page can be shown. For inspiration here, try scrolling as fast as possible through Google Images