Marking a group as 'pending deletion' should put all subgroups and projects into a read only state

Problem

When a group is marked as pending deletion, all subgroups and projects are currently displayed and function as usual with write access. This is misleading, since all the content contained within the group would be deleted together with the group marked for deletion.

For example:

• Users shouldn't be able add new members to a group or project that is pending deletion
• Subgroups and projects of a group pending deletion should not be able to execute the delete or restore options. This should happen at the level of the group that has been marked as pending deletion. => (dedicated issue) Marking a group as 'pending deletion' should di... (#549074 - closed)
• Users shouldn't be able to establish new group sharing links with a group or project that is pending deletion
• Users shouldn't be able to transfer a group or project that is pending deletion
• Users shouldn't be able to change the name, path, description or avatar of a group or project that is pending deletion
• Users shouldn't be able to change any group or project settings while it is pending deletion
• Users shouldn't be able to add new work items to a project that is pending deletion (epics, issues, etc.)
• Users shouldn't be able to add new labels to a group or project that is pending deletion
• Users shouldn't be able to create new milestones for a group or project that is pending deletion

Proposal

Ensure that all subgroups and projects are put into a read only state so that no actions can be taken unless the group is recovered.

Blocked actions should include deletion in subgroups and projects when a parent group is already pending deletion. Similar as is planned for archiving. A dedicated issue has been created for this.