Allow to share templates with users / projects

[codescape]

As a user I want to share existing templates to create subtasks with other users or projects. I can manage these sharing options on the user / project / global template administration view.

• User templates can be shared with other users (multiple users possible)
• Project templates can be shared with other projects (multiple projects possible)
• Global templates are visible to everyone (default) or can be restricted to be only used by workflow transitions

Considerations

Configuration: allow sharing for everyone or limit to users who have the global permission to share objects

Shared templates cannot be edited by the projects / users they are shared with. If we want this we would need to have two different sharing options (read-only / read-and-write).

Being able to restrict global templates this would also provide a good basis for #112.

Tasks

• collect feedback
• implement sharing for project templates
• implement sharing for user templates
• restrict sharing per configuration option
• documentation
• create separate issue for global templates