Show "loading" when payments balance is loading

Fix #3778
Fix #3785 or at least a gentle fallback

Auditors: @diracdeltas @bbondy

Test Plan:

Offline case:
1. Disable internet
2. Re/open Brave
3. Open Preferences -> Payments
4. Observe that account balance is "loading..." and not 0

Online case:
1. Open Brave
2. Open Preferences -> Payments
3. Observe account balance is accurate

Author: ayumi

app/extensions/brave/locales/en-US/preferences.properties

@@ -24,6 +24,7 @@ paymentsSidebarText2=All transaction IP addresses are anonymized with technology
 paymentsSidebarText3=Brave Bitcoin Wallets are provided through a partnership with:
 paymentsSidebarText4=Your contributions in the form of credit cards and bank cards are handled by:
 accountBalance=account balance
+accountBalanceLoading=loading...
 monthlyBudget=monthly budget
 status=status
 createWallet=create wallet

js/about/preferences.js

@@ -709,10 +709,16 @@ class PaymentsTab extends ImmutableComponent {
             <tr>
               <td>
                 <span id='fundsAmount'>
-                  {this.btcToCurrencyString(this.props.ledgerData.get('balance'))}
-                  <a href='https://brave.com/Payments_FAQ.html' target='_blank'>
-                    <span className='fa fa-question-circle fundsFAQ' />
-                  </a>
+                  {
+                  this.props.ledgerData.get('balance')
+                  ? <span>
+                    {this.btcToCurrencyString(this.props.ledgerData.get('balance'))}
+                    <a href='https://brave.com/Payments_FAQ.html' target='_blank'>
+                      <span className='fa fa-question-circle fundsFAQ' />
+                    </a>
+                  </span>
+                  : <span data-l10n-id='accountBalanceLoading' />
+                  }
                 </span>
                 {this.walletButton}
                 {this.paymentHistoryButton}