as_draws_df() and expand_grid()

Noteworthy changes include:

• replacing the depreciated posterior_samples() function with the new posterior::as_draws_df()-based workflow,
• replacing my incorrect use of tidyr::expand() with a more appropriate tidyr::expand_grid() workflow,
• adopting the new linewidth argument for several ggplot2 geoms, and
• refitting all models with brms 2.18.0.