Keep log files from projects after they disable ocaml-ci

[kit-ty-kate]

When a repository enables ocaml-ci, then disable it after some commit have been pushed the log files from those commits don't seem to be available anymore. For instance: https://ci.ocamllabs.io/github/ocaml-ppx/ppx/commit/40e5a35a4386d969effaf428078c900bd03b78ec/variant/(lint-fmt) previously showed some content but the ocaml-ci has been disabled for this project since and the page is now showing Error: Failed: Invalid GitHub repo "ocaml-ppx"/"ppx".

[talex5]

We disabled this previously (in #204). Since the project doesn't appear, things under it don't either.

It would be good to show a perma-link to the log from the main page (which only shows the latest build at the moment). Also, it would be nice to link to previous builds in the public UI (the admin UI already does this).