Account update email should mention the instance URL

Created by: unknwon

I reset my test account password on my dev instance, and it has email configured. Then I received the following notification:

Somebody (likely you) reset the password for the user * on Sourcegraph.

If this was not you please change your password immediately.

My email client shows it is coming from "[email protected]" (which is not really true).

I think the email body should mention which Sourcegraph instance (the externalURL) sent out the email, otherwise it is pretty confusing and could cause false alarm.