(core) Allow linking summary tables based on ref/reflist columns (except `group`)

Summary:
Relax the restriction in `selectBy.isValidLink` so that summary tables can be linked by a column like other tables, except the `group` column. See the discussion on https://grist.slack.com/archives/C0234CPPXPA/p1651773623256959 (the replies are on the following message) for more info on this decision.

Tweaked `LinkingState.ts` since linking with summary tables can now involve a column.

Test Plan: Added a new nbrowser test and fixture checking the options to select by given a summary table with a few ref/reflist columns. Manually tested the behaviour of each option.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3416

app/client/components/LinkingState.ts

@@ -96,7 +96,7 @@ export class LinkingState extends Disposable {
       }
     } else if (srcColId && isRefListType(srcCol.type())) {
       this.filterColValues = this._srcCellFilter('id', 'in');
-    } else if (isSummaryOf(srcSection.table(), tgtSection.table())) {
+    } else if (!srcColId && isSummaryOf(srcSection.table(), tgtSection.table())) {
       // We filter summary tables when a summary section is linked to a more detailed one without
       // specifying src or target column. The filtering is on the shared group-by column (i.e. all
       // those in the srcSection).

app/client/ui/selectBy.ts

@@ -69,8 +69,13 @@ function isValidLink(source: LinkNode, target: LinkNode) {
     return false;
   }
 
-  // summary table can only link to and from the main node (node with no column)
-  if ((source.isSummary || target.isSummary) && (source.column || target.column)) {
+  // cannot link to the somewhat special 'group' reflist column of summary tables
+  // because in most cases it's equivalent to the usual summary table linking but potentially slower,
+  // and in other cases it may cause confusion with overwhelming options.
+  if (
+    (source.isSummary && source.column?.colId.peek() === "group") ||
+    (target.isSummary && target.column?.colId.peek() === "group")
+  ) {
     return false;
   }