Reduce duplication of code in the various `plot_smooth()` methods

[gavinsimpson]

Intentionally, I have duplicated a lot of code in the various plot_smooth() internal methods, waiting until things had settled down a bit with the package before attempting any sort of code de-duplication or clean up.

With only the bs = "mrf" basis currently not supported, time is ripe to reduce the duplication across the plot_smooth() methods, such that I can use a single prepare_smooth_plot() across all the methods.

Investigate the feasibility of doing this and implement.