Using str_c caused issues when combining sb_dir with following directori...

Using `str_c` caused issues when combining `sb_dir` with following directories because there was no "/" separator.  In most situations this is resolved by using file.path instead of `str_c`, which also has the nice benefit of being faster.  In some situations where file.path would not be convenient due to also needing file extensions (mostly "r.data") a simple "/" put in the right place fixed the problem.

Prior to this, `runShinyBuilder` would not produce a working dashboard on Windows.  It now works on Windows but should be tested on Mac and Linux.