Fix inline code serialization in rich text editor

What does this MR do and why?

Fix inline code serialization in rich text editor

When an inline code block in rich text editor contains backticks, it breaks on serialization. Now such inline code blocks will be serialized using double backticks.

Changelog: fixed

Screenshots and steps

1. Type in rich text editor: ```js
2. Do not press Enter, else it will create a code block.
3. Select all.
4. Click "Insert code" button in the toolbar.
5. Save.

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