Mentions should not be expanded inside Markdown code blocks

As part of BSERV-5441 the front-end behaviour of mentions inside Markdown code blocks was fixed, however mentions are still expanded on the server side - so a preview/save will expand the mention inside a code block.

In an ideal world, our mentions do not get expanded inside Markdown code blocks because an "@" followed by a string can be a valid identifier, even if it does match a user, it should not be expanded.

For example, if the system contains a user with username "echo" and let's say their name is "Edwin Cho" the following code block would be expanded:

 
```
@echo off
cd C:\code
```

Edwin Cho off
cd C:\code

If possible, perhaps the Markdown conversion can be performed first and then ignore any text within a <code> or <pre> block.

This also affects code suggestions.