all: Add receive only folder type (#5027)

Adds a receive only folder type that does not send changes, and where the user can optionally revert local changes. Also changes some of the icons to make the three folder types distinguishable.

cmd/syncthing/mocked_model_test.go

@@ -29,6 +29,8 @@ func (m *mockedModel) Completion(device protocol.DeviceID, folder string) model.
 
 func (m *mockedModel) Override(folder string) {}
 
+func (m *mockedModel) Revert(folder string) {}
+
 func (m *mockedModel) NeedFolderFiles(folder string, page, perpage int) ([]db.FileInfoTruncated, []db.FileInfoTruncated, []db.FileInfoTruncated) {
 	return nil, nil, nil
 }
@@ -117,6 +119,10 @@ func (m *mockedModel) LocalSize(folder string) db.Counts {
 	return db.Counts{}
 }
 
+func (m *mockedModel) ReceiveOnlyChangedSize(folder string) db.Counts {
+	return db.Counts{}
+}
+
 func (m *mockedModel) CurrentSequence(folder string) (int64, bool) {
 	return 0, false
 }