Make tokens URL safe

Base64 encoding causes some issues when the token is URL encoded
as the = symbol is not a valid URL character. We replace any =
symbols with a - as this is valid in a URL, but is guaranteed
not to appear in a bade64 string. This fix ensures that Ghost
password resets work with mail providers such as Mailgun that
add their own tracking redirects

closes #3872 (for real this time)