Add `RememberMe::removeAllByUsername()` which deletes all rows from
`icingaweb_rememberme` for a given user. Call it from
`TwoFactorEnrollmentForm::onSuccess()` after a successful enroll so that
pre-enrollment cookies on other devices cannot bypass the newly required
second factor.
`TwoFactorController::configAction()` renders `TwoFactorEnrollmentForm`, which
shows a method-selector dropdown (autosubmit), the method-specific fieldset
contributed by `assembleEnrollmentFormElements()`, and either an "Enroll" or
"Unenroll" submit button depending on current enrollment state.
`AccountController`, `MyDevicesController`, and `NavigationController` each gain
a "Two-Factor Auth" tab.