fix(admin): shorten the receipt letter, show the account id

The receipt letter repeated what the receipt itself states. It now carries the
amount, the moment of payment with its offset, and the link — the receipt is
the document, and it opens without authentication, so there is nothing for the
two to disagree about. Subject reworded to name what it is.

The ledger and tax-export tables labelled the account link "card", meaning the
user card. On pages about payments that reads as a bank card, which is exactly
how it was misread. Both now show the first eight characters of the account id,
so the column carries data — two rows from the same buyer are visible at a
glance — while the link still holds the whole id.

backend/internal/adminconsole/templates/pages/ledger.gohtml

@@ -40,7 +40,7 @@ Money in: {{range .Totals.MoneyIn}}<strong>{{.}}</strong> {{else}}<span class="n
 {{range .Rows}}
 <tr>
 <td>{{.At}}</td>
-<td><a href="/_gm/users/{{.AccountID}}">card</a></td>
+<td><a href="/_gm/users/{{.AccountID}}"><code>{{.AccountShort}}</code></a></td>
 <td>{{.Kind}}</td>
 <td>{{.Money}}</td>
 <td>{{.ChipsDelta}}</td>