Create service for email verification

I have created the Verifier service.

The basic functionality to send the verify email, and create and log in users now exists.

Some test cases are still required.

Edited issue as the way I thought it would be implemented turned out not to be practical.

I was worried about the link being vulnerable to brute force attacks, so I added a second "secret" parameter to the route, and generate a random of string of characters. This is stored in in TempStore, and must match when the link is clicked.

I have added the following test cases:

• Test that entering a valid email address submits the form and displays a message
• Test an invalid link (not stored in TempStore) does not log in
• Test a link with incorrect secret does not log in
• Test an expired link does not log in, but does redirect to verify form and ask to reenter email
• Test a valid link does log in and redirects to the destination page

Merged change.

lostcarpark → credited cmlara → .

Crediting cmlara for assistance via Slack.

Automatically closed - issue fixed for 2 weeks with no activity.