Add charts (based on JFreeChart)

Currently only an expense report is supported, but more will come.
Also moved rest templates into own toplevel package.

financer-web-client/src/main/resources/templates/account/accountOverview.html

@@ -16,7 +16,9 @@
     </div>
     <div th:title="#{'financer.account-overview.tooltip.status.current-expenses'(${#temporals.format(periodStart)}, ${#temporals.format(periodEnd)})}">
         <span th:text="#{financer.account-overview.status.current-expenses}"/>
-        <span th:text="${#numbers.formatDecimal(currentExpenses/100D, 1, 'DEFAULT', 2, 'DEFAULT') + currencySymbol}"/>
+        <a th:href="@{/getAccountGroupExpensesCurrentPeriod}">
+            <span th:text="${#numbers.formatDecimal(currentExpenses/100D, 1, 'DEFAULT', 2, 'DEFAULT') + currencySymbol}" />
+        </a>
     </div>
     <div>
         <span th:text="#{financer.account-overview.status.recurring-transaction-due-today}"/>