Closes #93, "Why is the ||= removed? current_user method".  This adds
memoization back to current_user / Current.user, plus causes log out
when the current session is destroyed.

Closes #88.

Asserts cookie is http_only, secure, and same-site is "strict".
Closes #87.

Closes #86.

The issue is that signed tokens have a simple payload by default that
doesn't verify anything other than the record id and the token's
purpose.  This can lead to a security challenge as the token can be used
to confirm anything that has the same "purpose" for that record.

An example would be someone changing their email address to an email
address that they don't control.  Using one account, they change the
email to an email address that they control and get the confirmation
token.  They then change the email to one that they can't access and
use the token from the first request to "confirm" the second request.
The tokens can be used any number of times as long as they're used
before expiration.

With this change, the email address is included as plain text in the
request, as well as being used as part of the "purpose" in the token.
The second request fails because the plain text email address is used to
constrain the signed lookup.  If they change the plain text email
address in the link then the message will fail to be validated as the
"purpose" won't match.  Either way, the token is usable only for
confirming the original email from the token's creation.