Started port to Django 2.1, Python 3, Docker

This commit is a rough port to Django 2.1, Python 3 and a Docker based local
development setup. Tests fail/error but migrations and the web frontend are
already runnable. Task queue functionality is untested and translations seem to
have trouble.

Dockerfile

@@ -0,0 +1,31 @@
+FROM debian:stretch
+LABEL maintainer "Jan Dittberner <jan@dittberner.info>"
+
+RUN apt-get update \
+    && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+    dumb-init \
+    gettext \
+    git \
+    libpq-dev \
+    python3-dev \
+    python3-pip \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*.*
+
+VOLUME /srv/gnuviechadmin/media /srv/gnuviechadmin/static
+WORKDIR /srv/gnuviechadmin
+
+ENV LC_ALL=C.UTF-8
+ENV LANG=C.UTF-8
+
+RUN python3 -m pip install -U pip && pip3 install pipenv
+
+COPY gnuviechadmin.sh /srv/
+COPY Pipfile Pipfile.lock /srv/gnuviechadmin/
+
+RUN pipenv install --system --deploy --ignore-pipfile --dev
+
+EXPOSE 8000
+VOLUME /srv/gnuviechadmin
+
+ENTRYPOINT ["dumb-init", "/srv/gnuviechadmin.sh"]