Adjust layout of stop detail collection view

Modifies the collection views in the tour stop details footer to match the current design.
- Adds support for variable-sized collection view cells with paging and infinite scrolling.
- Allows paging layout to support arbitrary alignment options, rather than just centering the cells.
- Uses constraints in the nibs to specify exact content margins on the collection views, which are used to set the correct `contentInset` and page layout properties.