Allow selecting of multiple project, and multiple groups for templates
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### Problem to solve
Both the [Custom instance-level project templates](https://docs.gitlab.com/ee/user/admin_area/custom_project_templates.html) and the [Instance template repository](https://docs.gitlab.com/ee/user/admin_area/settings/instance_template_repository.html) only allow the selecting of one option. This means it is only possible to have a single project and a single group for instance wide templates.
### Proposal
Allow selection of multiple entries. Multiple groups/projects. This will allow many projects/groups to include many different project templates and file templates. Better organization/scope etc.
### Documentation
https://docs.gitlab.com/ee/user/admin_area/settings/instance_template_repository.html
https://docs.gitlab.com/ee/user/admin_area/custom_project_templates.html
### Links / references
[Internal ZD](https://gitlab.zendesk.com/agent/tickets/160840)
issue