Return email confirmation time from email entity

What does this MR do?

Issue: #26327 (closed)

Related to: #25077 (closed), but resolving that issue would take significant database changes which are outside the scope of this MR.

The API result from /user/emails (https://docs.gitlab.com/ee/api/users.html#list-emails) did not indicate whether an email is verified or not. This MR exposes the confirmed_at attribute, which returns a Time if the email has been confirmed and nil if not.

Before:

% curl --header "PRIVATE-TOKEN: $TOKEN" "http://127.0.0.1:3000/api/v4/user/emails"     

[
   {
      "id" : 1,
      "email" : "admin@example.com"
   },
   {
      "id" : 2,
      "email" : "new@example.com"
   },
   {
      "id" : 3,
      "email" : "test@example.com"
   }
]

After:

% curl --header "PRIVATE-TOKEN: $TOKEN" "http://127.0.0.1:3000/api/v4/user/emails"
[
   {
      "confirmed_at" : "2021-03-26T19:07:56.248Z",
      "email" : "admin@example.com",
      "id" : 1
   },
   {
      "email" : "new@example.com",
      "confirmed_at" : null,
      "id" : 2
   },
   {
      "email" : "test@example.com",
      "confirmed_at" : null,
      "id" : 3
   }
]

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #26327 (closed)