Send "welcome" email on first HKP upload

I've had a bunch of users create and upload keys, and wonder why they didn't receive some email about it. This MR introduces a simple "welcome" email. It is only sent when a key is first encountered, and points the user to the same site they would get to with the upload form (or the curl -T technique).

What's not nice about this, and the reason I hesitated a bit with this feature, is that it's not consistent. That is, it works differently the first time a key is uploaded than other times. This is not ideal in a number of scenarios, most notably when a user adds a uid and uploads their key again, expecting the same workflow like the first time. It also doesn't work for keys that were already known from the pool.

Still, this might improve the status quo. We'll just have to wait for user feedback