Focus login field when login page is loaded

[aetherfaerber]

Is your feature request related to a problem? Please describe.
When a user loads the login page to Kitodo they want to enter their login data. To do so, they must first select the field ”Benutzername” to do so.

Describe the solution you'd like
As the login page is rendered, the ”username” input field should be selected automatically so users can type away their username without switching between keyboard and mouse.

Describe alternatives you've considered
On page load, you cannot even tab into the username field but need to click anywhere on the page.

Additional context
There is not much to do on the login page anyway besides entering user credentials and logging in. So why not start doing it right away?