Replace deprecated String.prototype.substr()

String.prototype.substr() is deprecated (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr) so we replace it with slice() or substring() which work similarily but aren't deprecated.
Signed-off-by: Tobias Speicher <rootcommander@gmail.com>

src/renderer/views/components/mediaitem-square.ejs

@@ -145,7 +145,7 @@
                 }
             },
             revisedRandId() {
-                return Math.random().toString(36).replace(/[^a-z]+/g, '').substr(2, 10);
+                return Math.random().toString(36).replace(/[^a-z]+/g, '').slice(2, 10);
             },
             async isInLibrary() {
                 if (this.item.type && !this.item.type.includes("library")) {