[feature] Provide confirm email success page

[nemesifier]

Currently after confirming their email addresses, new users get redirected to a login form which if used doesn't take anywhere.

This could be confusing for end-users.

In order to not slow down the new release further we can opt for a solution which should be easy: provide a view which shows a page with the which just shows the success message and says "you can safely close this window".

Then we can just configure EMAIL_CONFIRMATION_ANONYMOUS_REDIRECT_URL and EMAIL_CONFIRMATION_AUTHENTICATED_REDIRECT_URL to point to this view in production (and we can add it to the default settings of this project and to the recommended settings in the README.