Fix all possible setting error related storages and added some tests (#23911)

Follow up #22405

Fix #20703 

This PR rewrites storage configuration read sequences with some breaks
and tests. It becomes more strict than before and also fixed some
inherit problems.

- Move storage's MinioConfig struct into setting, so after the
configuration loading, the values will be stored into the struct but not
still on some section.
- All storages configurations should be stored on one section,
configuration items cannot be overrided by multiple sections. The
prioioty of configuration is `[attachment]` > `[storage.attachments]` |
`[storage.customized]` > `[storage]` > `default`
- For extra override configuration items, currently are `SERVE_DIRECT`,
`MINIO_BASE_PATH`, `MINIO_BUCKET`, which could be configured in another
section. The prioioty of the override configuration is `[attachment]` >
`[storage.attachments]` > `default`.
- Add more tests for storages configurations.
- Update the storage documentations.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/migrations/v1_10/v96.go

@@ -53,7 +53,7 @@ func DeleteOrphanedAttachments(x *xorm.Engine) error {
 
 		for _, attachment := range attachments {
 			uuid := attachment.UUID
-			if err := util.RemoveAll(filepath.Join(setting.Attachment.Path, uuid[0:1], uuid[1:2], uuid)); err != nil {
+			if err := util.RemoveAll(filepath.Join(setting.Attachment.Storage.Path, uuid[0:1], uuid[1:2], uuid)); err != nil {
 				return err
 			}
 		}