Detail pages: add simple control to move to next / previous ICU version. Also next/previous platform

[sven-oly]

For comparing results from different versions and platforms, it would be helpful to have a quick control to navigate from a given detail page to other ICU versions for the platform.

Similar, add a control to navigate to other platforms with the same ICU version.

This could be a control that shows all appropriate targets, e.g., a pulldown menu.