Meets #11254: Copy project: Always error message "subproject of invalid"

[``* `#11254` Copy project: Always error message "subproject of invalid"``](https://www.openproject.org/work_packages/11254)
The code will require the existence of a project for wich the user has the :add_subprojects permission. This is not a necessity however. The user will need the following:
* Always: the :copy_projects permission in the current project
* In case the current project has a parent: the :add_subprojects permission in the parent project 

<pre>
current_user.allowed_to?(:copy_project, @project) && (@project.parent.nil? || current_user.allowed_to?(:add_subprojects, @project.parent))
</pre>
should do that. 

This could then be moved to the Project model, e.g. #copy_allowed? 

This would then also replace the duplicate check currently placed within the view
<pre>
link_to_if_authorized
</pre>
and
<pre>
@copy_allowed
</pre>
with one
<pre>
@project.copy_allowed?
</pre>