Needs proper message that you can't add version/milestone without project

[Cinc-th]

Default behaviour after installation is to not allow any version/milestone without a project. If there are no projects created yet you see an empty project select box and can't add any versions or milestones.

This is confusing because the help text is just saying

Please chose the projects for which this item will be selectable. Without a selection here no restrictions are imposed.

which isn't correct.

Todo

• Change the help text to make clear that you must chose a project
• If there are no projects yet then hide the project table and display an appropriate message
• Keep the help text in case version_without_project = True is set (similar for milestones)

Hint

The following Settings in trac.ini may be used to control behaviour:

[simple-multi-project]
version_without_project = True
single_project_versions = True

[simple-multi-project]
milestone_without_project = True
single_project_milestones = True

[Cinc-th]

In 18016:

Show a message on the milestone and version admin pages when no projects are defined or open.

Refs #13966

[Cinc-th]

In 18017:

Show a message on the component admin page when no projects are defined or open. Follow up to [18016].

This fixes an exception caused by legacy component filter code. This code is not using the base class implemented for the admin page handling yet.

Refs #13966

[Cinc-th]

In 18018:

Show proper information regarding project selection depending on configuration.

Closes #13966