Make public and rename repo

As per fdroid-website#101, we can download and copy into the webroot when running the f-droid.org CI task. That requires the repository to be made public though. Given this will be a permanent feature of the F-Droid GitLab group, we should probably rename it to something more appropriate too, such as fdroid-website-legacy-forum (so it sits next to fdroid-website in the list of projects).

The rationale for keeping it permanently here rather than in the fdroid-website project itself is:

• It is quite large, and would require potential contributors to download almost 10MiB instead of just a few MiB to contribute to the website project.
• Essentially no developers require this while working on the website. As such, the extra size that it adds is for no benefit.