As the new-age Python package manager, Poetry brings a lot
of good additions to the table. It allows us to more easily
deal with virtualenvs for the project and resolve dependencies.

As of this commit, `requirements.txt` is replaced by Poetry,
configured at `pyproject.toml`.

In Docker and GitLab, we currently use Poetry in a root fashion.
We should work toward purely using virtualenvs in Docker, but,
for now we'd like to move forward with other things. The project
can still be installed to a virtualenv and used on a user's system
through Poetry; it is just not yet doing so in Docker.

Modifications:

* docker/scripts/install-deps.sh
    * Remove python dependencies.
* conf/config.defaults
    * Script paths have been updated to use '/usr/bin'.
* docker/git-entrypoint.sh
    * Use '/usr/bin/aurweb-git-auth' instead of
      '/usr/local/bin/aurweb-git-auth'.

Additions:

* docker/scripts/install-python-deps.sh
    * A script used purely to install Python dependencies with Poetry.
      This has to be used within the aurweb project directory and
      requires system-wide dependencies are installed beforehand.
    * Also upgrades system-wide pip.

Signed-off-by: Kevin Morris <kevr@0cost.org>