Revert "Rename folders to be plural because it bothers me.."

This reverts commit 6c5ede79d143f8d3e4be42970768b269edea6c2f.

Conflicts:
	system/cms/core/MY_Controller.php
	system/cms/modules/streams_core/src/Pyro/Module/Streams_core/EntryModel.php
Singular
Merge branch 'refs/heads/2.3/develop' into 3.0/develop

Conflicts:
	system/cms/core/Admin_Controller.php
	system/cms/modules/streams_core/field_types/image/src/Pyro/FieldType/Image.php
	system/cms/modules/streams_core/field_types/relationship/src/Pyro/FieldType/Relationship.php
	system/cms/modules/users/controllers/admin.php