Don't use simpleMDE editor on mobile devices for 1.13 (#14029)

* Don't use simpleMDE editor on mobile devices

simpleMDE doesn't work properly on mobile devices -- We've replaced it with the slightly more working easyMDE in 1.14 but since that change can't be backported to 1.13 we will just disable the editor on mobile here.

* make isMobile function per code review -- disable simpleMDE for code review and replies

* Fix issue with plain text and wiki

Co-authored-by: silverwind <me@silverwind.io>

web_src/js/utils.js

@@ -19,6 +19,11 @@ export function isDarkTheme() {
   return document.documentElement.classList.contains('theme-arc-green');
 }
 
+// returns if mobile device
+export function isMobile() {
+  return /Mobi/.test(navigator.userAgent);
+}
+
 // removes duplicate elements in an array
 export function uniq(arr) {
   return Array.from(new Set(arr));