suffers from the same problem as window.btoa - can't handle unicode strings

https://bugzilla.redhat.com/show_bug.cgi?id=1527316

`window.btoa("sněhulák")` fails on InvalidCharacterError: String contains an invalid character

because it only expects latin1 chars

`$base64.encode` from `angular-base64` has the same problem.

We need to base64 encode the login:password pair even when the password uses non-latin1 chars :)

Implementation adapted from https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding#Solution_2_%E2%80%93_rewrite_the_DOMs_atob()_and_btoa()_using_JavaScript's_TypedArrays_and_UTF-8

https://bugzilla.redhat.com/show_bug.cgi?id=1527316

…code

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1527316

… the encoder

and explicitly say window.TextEncoder for when it doesn't exist