Add 10min timeout to Push Mirrors
See https://gitlab.com/gitlab-com/infrastructure/issues/1345.
Pull mirrors have a timeout, as can be seen here: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/lib/gitlab/shell.rb#L129 and here: https://gitlab.com/gitlab-org/gitlab-shell/blob/master/lib/gitlab_projects.rb#L214.
Push mirrors don't currently: https://gitlab.com/gitlab-org/gitlab-ee/blob/master/lib/gitlab/shell.rb#L310 and https://gitlab.com/gitlab-org/gitlab-shell/blob/master/lib/gitlab_projects.rb#L121.
We should implement the same timeout mechanism there, and update the docs and "Mirrors" page to say push and pull mirrors have a 10 minute timeout.