Resolve auto at plot time

[cpsievert]

• f446627 fixes a simple oversight made in #68
• 9776fbd makes sure not to call auto_resolve_theme() outside of a plot-time context
  • When font = "auto", auto_resolve_theme() wants to make a check for is_rstudio_gd() which calls infer_device(), which is really only a valid thing to do at plot-time.
  • shiny needs to do the same (https://github.com/rstudio/shiny/pull/3178)
• c93aad1 removes functions that were deprecated months ago