v2.0.10 - the dashboard's Add a submodule section is folded by default

The dashboard's "Add a submodule" card is now collapsed by default, the same fold the commit log already used. Click its heading to open it; the choice is kept per user (the preference admin_git_manager_add_submodule) so it survives a reload. It opens on its own if an add just failed validation, so the error is never hidden behind the fold, and it still works as a plain details element with JavaScript off.

No functional change from v2.0.9. v2.0.9's commit messages and release notes wrongly carried a co-authorship trailer that this project's history never uses; that release cannot be edited or withdrawn, so this release corrects it going forward. Use v2.0.10.