Re-implemented guessing of next document id.

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

frontend/src/routes/document/View.svelte

@@ -2,7 +2,7 @@
     import { onMount }       from 'svelte';
     import { useTinyRouter } from 'svelte-tiny-router';
 
-    import {api}             from '../../urls.svelte.js';
+    import { api, post }     from '../../urls.svelte.js';
     import { error, yikes }  from '../../warn.svelte';
     import { t }             from '../../translations.svelte.js';
     import { user }          from '../../user.svelte.js';
@@ -29,11 +29,7 @@
 
     async function addPosition(selected){
         const url  = api(`document/${doc.id}/position`);
-        const resp = await fetch(url,{
-            method      : 'POST',
-            credentials : 'include',
-            body        : JSON.stringify(selected)
-        });
+        const resp = await post(url,selected);
         if (resp.ok){
             doc.positions = await resp.json();
             yikes();