Conflict with automated markup links prevents adding new checklist items

If a checklist item has a trigger for an automatic markup link, then no items can be added directly after it. The automatic markup links code is specific to WordPress.com, but other users could experience similar conflicts.

Steps to reproduce:

1. Create a list inside a post
1. Click on one of the entries, then on the `Add New After` button
1. Enter the following as the new item content and submit it: `text with a trigger for automated markup links like #121-gh-wp-super-cache foo bar`
1. Click on the new item from the previous step, then on the `Add new After` button
1. Enter this content and submit it: `just regular text without anything special`

The request in the final step will fail. If you try to add it after an item that doesn't have the trigger it will work. It also works if the `[issue https://github.com/automattic/wp-super-cache/pull/121]` shortcode is used instead of the trigger for the automated link.

Start by reproducing the final Add New After request with an item containing an automatic markup-link trigger, then trace how checklist insertion handles that trigger and the request failure. Done means adding a regular item after a triggered item succeeds, while insertion after ordinary items and the documented shortcode behavior remain working.

Written by the indexing model from the issue text.