2.12 build: replace harrah/browse with sbt/sxr

[SethTisue]

the latter receives light maintenance, the former is abandoned

(I discovered the latter only after putting together the changes in harrah/browse#27)

[SethTisue]

@eed3si9n I'm going to close this ticket as not worth the effort unless/until sxr maintainers or community start actively supporting Scala 2.12