Fix issue comment submit button disabled on GFM paste

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/40373

When copying some text from another note and paste,
it goes through `copy_as_gfm` `paste` handler
that calls `e.preventDefault()` which stops things.
But then when inserting the text, we manually trigger
an `input` event with jQuery which doesn't seem to be
picked up by Vue `v-model`.

Using copy/paste trick from https://stackoverflow.com/a/41046276/796832

app/assets/javascripts/lib/utils/common_utils.js

@@ -190,7 +190,7 @@ export const insertText = (target, text) => {
   target.selectionStart = target.selectionEnd = selectionStart + insertedText.length;
 
   // Trigger autosave
-  $(target).trigger('input');
+  target.dispatchEvent(new Event('input'));
 
   // Trigger autosize
   const event = document.createEvent('Event');

changelogs/unreleased/40373-fix-issue-note-submit-disabled-on-paste.yml

+---
+title: Fix Issue comment submit button being disabled when pasting content from another
+  GFM note
+merge_request: 15530
+author:
+type: fixed