Show cash + holdings value for investment account view (#1046)

* Handle missing tickers in security price syncs

* Show combined cash and holdings value on account page

* Improve partial locals

app/helpers/account/cashes_helper.rb

@@ -0,0 +1,13 @@
+module Account::CashesHelper
+  def brokerage_cash(account)
+    currency = Money::Currency.new(account.currency)
+
+    account.holdings.build \
+      date: Date.current,
+      qty: account.balance,
+      price: 1,
+      amount: account.balance,
+      currency: account.currency,
+      security: Security.new(ticker: currency.iso_code, name: currency.name)
+  end
+end