Create email notifications when assigning issues to a user

... So that users can actually know right away about their new issues.

This introduces a dependency on the Email Notifications plugin (as could be expected), but I believe it is installed by default, so it should not be an issue.

While I was at it, I also created a "batch update" notification: a user can receive one (and only one) email notification when a bunch of their issues has been modified (or fixed) in the analysis. I'm not sure this really belongs in this plugin, not being directly related to assigning issues, so I kept it in its own commit.