Improve thumbnail handling for oral histories

fixes #274

Explicitly specify oral history thumbnail images
Avoid rendering thumbnails for oral history titles
Remove redundant oral history thumbnails

this strategy is more in line with how we do slideshows (specifying a path to a thumbnail image for each slide instead of trying to auto-detect it). it fixes a couple of problems:

when experiences are on two wallscreens, we would otherwise need to duplicate all of the thumbnail images so that each version of the experience would get its own set of thumbnails. this way, we just specify the path to the thumbnail once and store it anywhere.
people who are interviewed more than once in an oral history don't need an identical copy of their thumbnail with a slightly changed named (e.g. john-smith-2); they can just point to the original thumbnail.
oral history titles, which don't need thumbnails at all, no longer need dummy placeholder thumbnails to satisfy the template logic.
oral history titles don't need to insert a special class or use any CSS; the template detects if an item is a title and accordingly renders the minimum amount of HTML (no image).

sul-dlss / wallscreens

Improve thumbnail handling for oral histories #275