Ref T7803. Removes some getReversePaging().

This also fixes null column handling, by adding an explicit 'null' key with possible values "head" (put NULL before other values) or "tail" (put NULL after other values).

Maniphest has some glitchiness in paging through NULLs right now, but I believe it's all pre-existing and will be resolved when it fully converts. Diffusion is fully converted and pages through NULL correctly.

• Failed to identify any reason for ChangesetQuery to reverse paging.
• Paged thorugh Diffusion.
• Paged through Maniphest.
  • Maniphest has some issues when paging inside a NULL section, but these issues are preexisting and will be resolved later in this change sequence.