Flash sides should control whether previous/next crosses act boundaries

[towerofnix]

It's useful to be able to use the nav bar (or shift+N, shift+P) to cross to flashes from adjacent acts of Homestuck, but we use acts as more general groupings of flashes elsewhere through the wiki.

Side: Fan Adventures
Navigate Across Act Boundaries: false

Thinking this should be sufficient. Doesn't seem like we need to introduce additional rules for finer-grain control.

Behavior details:

1. If this is the first flash in an act, and the current flash's side or the previous flash's side (if a different side) is Navigate Across Act Boundaries: false, then do not show a "Previous" control.
2. If this is the last flash in an act, and the current flash's side or the next flash's side (if a different side) is Navigate Across Act Boundaries: false, then do not show a "Next" control.

That is, if either flash of a connection between acts belongs to a side with Navigate Across Act Boundaries: false, then the connection should not be shown (as navigation controls).