[feature] Client API endpoints + v. basic web view for pinned posts (#1547)

* implement status pin client api + web handler

* make test names + comments more descriptive

* don't use separate table for status pins

* remove unused add + remove checking

* tidy up + add some more tests

internal/validate/status_test.go

@@ -70,7 +70,6 @@ func happyStatus() *gtsmodel.Status {
 		Likeable:                 testrig.TrueBool(),
 		ActivityStreamsType:      ap.ObjectNote,
 		Text:                     "Test status! #hello",
-		Pinned:                   testrig.FalseBool(),
 	}
 }