Upload staticfiles to S3 (#12220)
Django 4.2 deprecated the way that storages are handled and introduced
`STORAGES` setting. Then Django 5.2 removed the old pattern and our
staticfiles were broken after the upgrade.
This commit makes a very simple usage of `STORAGES` to define the
`staticfiles` storage so these files go to S3.
Note that I'm not changing the underlying configuration of the other
storages we use (build media, build tools, etc) for now; but we could
upgrade our code to follow the new pattern if we want to.
I tested this from `web-extra` and now the message is different:
```
docs@web-extra-i-0ef973eb0da33af39(org):~/checkouts/readthedocs.org$ django-admin collectstatic
You have requested to collect static files at the destination
location as specified in your settings.
This will overwrite existing files!
Are you sure you want to do this?
Type 'yes' to continue, or 'no' to cancel: yes
329 static files copied.
docs@web-extra-i-0ef973eb0da33af39(org):~/checkouts/readthedocs.org$
```
Besides, I can see the ext-theme files in the static S3 bucket now.
Closes #12211
