Minimize the default set of extensions

# Background

We have a number of dependencies listed in `requirements.txt` that no longer align with best practices. Further, any extensions used in the Starter Pack should have observable value across the entire organization. This doesn't mean that teams can't incorporate additional extensions, only that the Starter Pack should represent a "standard set".

# Request

Remove the following extensions from `requirements.txt`:

* `sphinx-tabs`: We already depend on `sphinx-design`, which is more robust in several ways. Using both extensions in tandem is also problematic for Ulwazi adoption.
* `sphinx-contributor-listing`: This extension fell out of favor some time ago, and I believe we already removed the associated template material from the Starter Pack.
* `sphinx-filtered-toctree`: The `filtered-toctree` directive is part of a larger feature that was originally implemented in LXD. In isolation, the extension offers no valuable functionality for users. Despite this, it still sees erroneous usage, with many using it as a normal `toctree`.
* `sphinx-last-updated-by-git`: This was originally included as a prerequisite for `sphinx-sitemap`. However, this is no longer necessary, as `sphinx-sitemap` now states this as a dependency and [sets it up by default](https://github.com/jdillard/sphinx-sitemap/blob/master/sphinx_sitemap/__init__.py#L62).
* `sphinx-ubuntu-images`: This extension isn't widely used. It will still be available for the projects that do use it, but it shouldn't be included in our minimal suite of extensions.