Adding a welcome email for new users.

Added the infrastructure necessary to support sending a email to
users when they sign up, either locally or by using an OAuth
provider. Also added a basic welcome message for new users.

Fixes #374
Added metrics for assigned/completed/archived tasks.

Previously, the user page only included open/closed metrics.
Now the results have been differentiated to show the different
types. Additionally, empty cells now show empty instead of 0/0.

Fixes #390
Addressing concerns with prior commit.

* Adding a link to the system to the text notification.
* Removed unnecessary lookup for email address from preflight.
* Moved dup welcome email code into a helper method.
Made the task loop async, as suggested.

* The task counts per user are now async instead of looping
over the results.
* The volunteer column needed to support the open/assigned/completed/archived
breakdown.
Merge pull request #403 from DavidEBest/devel-useradminextended

Added metrics for assigned/completed/archived tasks.
Removing unnecessary parameter to email generation.
Merge pull request #401 from DavidEBest/devel-welcomeemail

Adding a welcome email for new users.
increment to 0.0.23
Merge pull request #406 from 18F/master

increment to 0.0.23