Dashboard: replace Base64 with atob/btoa

This replaces the vendored Base64 code we're using with thin wrappers on top of the builtin `atob` and `btoa` functions that are in Firefox and Chrome, as well as IE 10 and newer. The wrappers are capable of safely roundtripping both Unicode strings and mojibake.

This resolves the issues with the Base64 code we were using misdecoding certain regions of Unicode characters.