Once mementos are displayed, there is no way to hide them

[machawk1]

In the main replay interface, selecting the number of mementos shows a list of available mementos. Once they are displayed, this option is cached so when a user returns to the interface, the list is not reverted to hidden.

However, once the mementos are displayed, clicking the same option does not revert them to hidden. The ability to toggle ought to be available instead of the option having a non-reversible effect.

Aside: This made testing #495 more difficult, as the process of displaying mementos causes drastic UI changes on mobile that are irreversible without clearing cookies(?)/cache.