Login form: Increase time the result message is shown

As a user I want to be able to read the result message after I log in as a frontend user in order to be able to understand if the login worked correctly.

Acceptance criteria

• The message after submitting the login form is shown for a longer time. OR
• A "Confirm" button is shown, so that the user needs to close the message actively.

More information

The result message is shown in a flash message which only very briefly appears on the website. It's very hard to read in that short time.