Improve compression by converting the u8array to base64

[andinus]

Earlier we were converting it to string with .toString() but that would add ',' after each element and that was starting to add up.

According to my tests on 100 paragraphs of lorem ipsum, containing 20559 characters.

• Compressing (and using .toString()) this string and encrypting it, the url was about 31268 characters.

• Storing it without compression, the url was about 27812 characters.

In this case, compression was not helping us.

• Storing it by converting u8array to base64, the url was about 11748 characters.

That's more than 3x improvement.

[andinus]

This change will break previously generated URLs.