using onScrollToBottom without fixed headers

[fran-worley]

A potential fix for this was raised here:
#674

However this doesn't seem to address the underlying issue that it appears possible to use onScrollToBottom without fixed headers when in reality this simply doesn't work with the current implementation.

We should either:

1. Support this out the box and ensure our ember infinity scrollContent selector is correct even when lt-scrollable isn't being rendered (not sure this is possible).
2. Support this by updating our useVirtualScrollbar property to be true when onScrollToBottom is rendered.
3. Raise a more descriptive error and update the docs to make it clear that you can only use onScrollToBottom event with fixed headers/footers.