Option to show issues as project dashboard even if repository is enabled

Description

We should be able to configure a specific project dashboard to show issues instead of README and/or files. If we want to use a project only as an issue-tracker and we disabled the file repository, the issues are already shown as a dashboard.

Currently you are unable to use templates because you need to add them to the repository which will cause GitLab to show README and/or files as a dashboard for a project.

/cc @filipa