'Convert a FeatureProfile object to its base64 signature.
>>> profile.to_signature()
\'=////////Hw==.53.9\''

def to_base64_signature(self):

    self.bitfield = FeaturesBitField.from_list(self.values())
    return ('=%s.%s.%s' % (self.bitfield.to_base64(), len(self), settings.APP_FEATURES_VERSION))
