Fix Javascript issue on non sqlite databases

When a non sqlite database is used, loading the admin interface fails
because the backup button is not generated.
This PR is solves it by checking if the elements are valid.

Also made some other changes and fixed some eslint errors.
Showing `_post` errors is better now.

Update jquery to latest version.

Fixes #3166

src/static/scripts/admin_diagnostics.js

@@ -1,4 +1,6 @@
 "use strict";
+/* eslint-env es2017, browser */
+/* global BASE_URL:readable, BSN:readable */
 
 var dnsCheck = false;
 var timeCheck = false;
@@ -65,7 +67,7 @@ function checkVersions(platform, installed, latest, commit=null) {
 
 // ================================
 // Generate support string to be pasted on github or the forum
-async function generateSupportString(dj) {
+async function generateSupportString(event, dj) {
     event.preventDefault();
     event.stopPropagation();
 
@@ -114,7 +116,7 @@ async function generateSupportString(dj) {
     document.getElementById("copy-support").classList.remove("d-none");
 }
 
-function copyToClipboard() {
+function copyToClipboard(event) {
     event.preventDefault();
     event.stopPropagation();
 
@@ -208,12 +210,18 @@ function init(dj) {
 }
 
 // onLoad events
-document.addEventListener("DOMContentLoaded", (/*event*/) => {
+document.addEventListener("DOMContentLoaded", (event) => {
     const diag_json = JSON.parse(document.getElementById("diagnostics_json").innerText);
     init(diag_json);
 
-    document.getElementById("gen-support").addEventListener("click", () => {
-        generateSupportString(diag_json);
-    });
-    document.getElementById("copy-support").addEventListener("click", copyToClipboard);
+    const btnGenSupport = document.getElementById("gen-support");
+    if (btnGenSupport) {
+        btnGenSupport.addEventListener("click", () => {
+            generateSupportString(event, diag_json);
+        });
+    }
+    const btnCopySupport = document.getElementById("copy-support");
+    if (btnCopySupport) {
+        btnCopySupport.addEventListener("click", copyToClipboard);
+    }
 });