Backport archive mode for camp23

src/core/models/events.py

src/core/templatetags/hub_absolute.py

src/hub/settings/base.py

src/plainui/jinja2.py

src/plainui/jinja2/plainui/assembly.html

src/plainui/jinja2/plainui/board.html

src/plainui/jinja2/plainui/components/function_btns.html

src/plainui/jinja2/plainui/components/header_buttons.html

src/plainui/jinja2/plainui/dereferrer.html

src/plainui/jinja2/plainui/login.html

src/plainui/jinja2/plainui/room.html

src/plainui/jinja2/plainui/signup.html

src/plainui/jinja2/plainui/static_page.html

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

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