Fix sign error in AdjointStabilization classes

This does still turn the examples/convection_cell results into garbage.

However, refinement (especially boundary layer refinement) fixes those results (they converge towards a reasonable looking solution, and if I bump up mu&k then the solution resembles an unstabilized solution).

With the wrong sign, coarse grids look better superficially, but the flow behavior seems to be incredibly sensitive to T_ref, which in theory should have no effect other than to add or subtract a vertical pressure gradient.

We can discuss this for a bit before merging.