Add tags for projects

src/backoffice/templates/backoffice/project/create_edit.html

src/core/forms.py

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

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

src/core/models/projects.py

src/core/models/tags.py

src/plainui/jinja2/plainui/components/list_assemblies.html

src/plainui/jinja2/plainui/components/list_projects.html.j2

src/plainui/jinja2/plainui/components/list_rooms.html.j2

src/plainui/jinja2/plainui/tag.html.j2

src/plainui/views/general.py

src/plainui/views/projects.py