improve invitations UI

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

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

src/backoffice/static/backoffice.css

...ice/templates/backoffice/components/invitations_list.html

...koffice/templates/backoffice/invitations/create_edit.html

src/backoffice/templates/backoffice/invitations/detail.html

src/backoffice/templates/backoffice/invitations/list.html

src/backoffice/templates/backoffice/teams/list.html

src/backoffice/views/invitations/__init__.py

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

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