Dashboard: replace Base64 with atob/btoa

The current JavaScript-based base64-encoding library we're using in
Archivematica and the storage service mishandles certain Unicode
codepoints. This corrupts certain strings when decoding/encoding, and
makes it impossible to roundtrip all data. For example:

	Base64.decode("8J+Yhy3wn5iP") // => "ߘ筰", not "😇-😏"

This replaces the vendored Base64 library with thin wrappers around the
native JavaScript atob/btoa functions. Those functions are available in
Firefox, Chrome, Safari, and Internet Explorer 10+.

Fixes #7698.