Update plainui message templates

src/plainui/jinja2/plainui/personal_message_list.html.j2

src/plainui/jinja2/plainui/personal_message_send.html.j2

src/plainui/jinja2/plainui/personal_message_show.html.j2

src/plainui/locale/de/LC_MESSAGES/django.po

src/plainui/locale/en/LC_MESSAGES/django.po