Evaluate copyText lazily

Summary:
`copyText` is generated when building rows for table, and is typically filled by JSON stringifying the incoming data. That is a pretty expensive process that could be done lazily. As shown in later diffs, this reduces the generation of rows for tables from ~18ms to ~3ms, which makes rendering a lot smoother.

(n.b. fixing the code duplication in the managedtables is part of the component lib plan)

Reviewed By: passy

Differential Revision: D21929660

fbshipit-source-id: 67cc69945e2bb28a6462a9d9ab765e33ced89378

desktop/app/src/ui/components/table/ManagedTable.tsx

@@ -575,9 +575,10 @@ export class ManagedTable extends React.Component<
     }
     return this.props.rows
       .filter((row) => highlightedRows.has(row.key))
-      .map(
-        (row: TableBodyRow) =>
-          row.copyText || this.getTextContentOfRow(row.key).join('\t'),
+      .map((row: TableBodyRow) =>
+        typeof row.copyText === 'function'
+          ? row.copyText()
+          : row.copyText || this.getTextContentOfRow(row.key).join('\t'),
       )
       .join('\n');
   };