Add option to use per-page fit modes.

## Problem

We would like to be able to set a per-page fit mode that will override the global fit mode on the scroll list.

## Solution

Do it.

## Tests

Added.

## QA

Tests pass. To test the code, change the scroll list demo such that the items inserted into the list have a `fit` attribute that is set to "auto", "height", or "width" (don't make them all the same. When you run the demo now, in peek or single mode, the default zoom will change from item to item. The `fitUpscaleLimit` parameter to the scroll list also has to be huge (1000 or something). There is a branch you can use for testing that already has these changes made. It is called `test_per_page_fit_mode` (https://github.com/Workiva/wf-uicomponents/tree/test_per_page_fit_mode).