UI messaging for slow key generation

[jimCresswell]

Added messaging to the login form to indicate that key generation may take a long time on slow devices. This does not fix the slow key generation but does adjust expectations so it should not be frustrating. Fix #40.

[jimCresswell]

I'd prefer to keep it simple and not introduce a delay, I don't think anyone is going to mind seeing a message saying it might take time and then getting the keys fast, they'd probably feel quite pleased with themselves. On the other hand diligently waiting five seconds and then being told you have to wait more might be annoying.

If we do want to introduce a delay in the messaging I would prefer to do it in CSS and work the Angular classes, something like

.my-div.ng-hide-remove {
  transition:3s step-end all;
}
.my-div {
  height: auto;
  opacity:1;
}
.my-div.ng-hide {
  height: 0;
  opacity: 0;
}

@zhukov , any thoughts?

[zhukov]

Thanks for the PR. I think that css animation is reasonable. I will merge this soon.