Überarbeite Login, Sign-up und PW-Reset

src/plainui/jinja2/plainui/header.html.j2

src/plainui/jinja2/plainui/login.html.j2

src/plainui/jinja2/plainui/password_change.html.j2

src/plainui/jinja2/plainui/password_reset_complete.html.j2

src/plainui/jinja2/plainui/password_reset_confirm.html.j2

src/plainui/jinja2/plainui/password_reset_done.html.j2

...i/jinja2/plainui/registration/password_reset_form.html.j2

src/plainui/jinja2/plainui/signup.html.j2

src/plainui/jinja2/plainui/signup_done.html.j2

src/plainui/styles/components/_landingpage.scss

src/plainui/styles/utils/_layout.scss