Re-implemented guessing of next document id.

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

frontend/src/routes/document/TemplateSelector.svelte

@@ -1,8 +1,8 @@
 <script>
-    import {onMount} from 'svelte';
+    import {onMount}   from 'svelte';
 
-    import {api}     from '../../urls.svelte.js';
-    import {t}       from '../../translations.svelte.js';
+    import {api, post} from '../../urls.svelte.js';
+    import {t}         from '../../translations.svelte.js';
 
     let {
         caption,
@@ -16,11 +16,7 @@
 
     async function loadTemplates(){
         const url = api('document/templates');
-        var resp = await fetch(url,{
-            credentials : 'include',
-            method      : 'POST',
-            body        : JSON.stringify({company:company})
-        });
+        var resp = await post(url,{company:company});
         if (resp.ok){
             templates = await resp.json();
         } else {