Better token location

[nathancday]

We've stumbled on path issues by storing tokens in the R session's HOME directory. But by stumbling we found a new, cleaner way to store tokens.

For what boxr is doing (storing a token), you probably want to use either tools::R_user_dir(..., which = "cache") (introduced in R 4.0.0) or rappdirs::user_cache_dir()) (would work on earlier versions of R) as the location. That's sort of the morally right place to put the token anyway and it relieves you of your Windows vs everything else dilemma.

Back porting this function to pre-4.0 feels a bit overkill IMO, but we could target this change in token location for a later R/boxr release version. I think the code to handle token paths is only in a few auth/read functions and should be stable.