(core) Fix exports to CSV/XLSX/etc when data is restricted by access rules

Summary:
- The issue manifested as error "Cannot read property '0' of undefined" in some
  cases, and as "Blocked by table read access rules" in others (instead of
  limiting output to what's not blocked)
- Goes deeper: exports weren't respecting metadata censoring.
- The fix changes exports to use censored metadata, which addresses both errors above.
- Includes an improvement to column ordering in XLSX exports.

Test Plan: Add a server test for CSV and XLSX exports with access rules

Reviewers: paulfitz, georgegevoian

Reviewed By: paulfitz, georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3615

app/server/lib/GranularAccess.ts

@@ -2170,6 +2170,13 @@ const dummyAccessCheck: IAccessCheck = {
 
 /**
  * Manage censoring metadata.
+ *
+ * For most metadata, censoring means blanking out certain fields, rather than removing rows,
+ * (because the latter was too big of a change). In particular, these changes are relied on by
+ * other code:
+ *
+ *  - Censored tables (from _grist_Tables) have cleared tableId field. To check for it, use the
+ *    isTableCensored() helper in app/common/isHiddenTable.ts. This is used by exports to Excel.
  */
 export class CensorshipInfo {
   public censoredTables = new Set<number>();