feat: reduce initial requests on wallet page (#2335)

* feat: reduce initial request on wallet page
- refactor allowed_currencies into function to use in generic and api.
- remove currencies request in frontend move it to generic
- dont request balance on first payments fetch

lnbits/templates/macros.jinja

@@ -1,6 +1,9 @@
 {% macro window_vars(user, wallet) -%}
   <script>
     window.extensions = {{ EXTENSIONS | tojson | safe }};
+    {% if currencies %}
+      window.currencies = {{ currencies | tojson | safe }};
+    {% endif %}
     {% if user %}
       window.user = {{ user | tojson | safe }};
     {% endif %}