[Markdown] Newline in post is a character too late when a code block is used

**Current Behavior**
The codeblock causes a post to have a horizontal scroll bar on mobile when added as the last part of a line.

**Steps to Reproduce**
1. Add a code block to a post which ends at the last character in a line (the "`" character is one character past where the new line should be)

**Expected Behavior**
The code block is moved to a new line

**Screenshots**
[](https://imgur.com/a/4sHph5X)

**Environment**
- Website URL: https://discuss.flarum.org
- Browser: Chrome 71 mobile

**Additional Context**
You can see the bug here: https://discuss.flarum.org/d/11006-twofactor-by-reflar/34

Reproduce the issue in the linked discussion using Chrome 71 mobile, with a post whose final content is a code block. Start by tracing the Markdown rendering for code blocks and verify that the block moves to a new line without creating horizontal scrolling; done means the expected line break appears on mobile.

Written by the indexing model from the issue text.