[skeb] implement 'include' option (#6558 #7267)

- split 'user' extractor into 'works' & 'sent-requests' extractors
- use BASE_PATTERN & USER_PATTERN
- use self.groups

scripts/supportedsites.py

@@ -380,6 +380,7 @@ SUBCATEGORY_MAP = {
     "skeb": {
         "following"      : "Followed Creators",
         "following-users": "Followed Users",
+        "sent-requests"  : "Sent Requests",
     },
     "smugmug": {
         "path": "Images from Users and Folders",