Fix issue in RTE related to adding text before a mention

What does this MR do and why?

Fix issue in RTE related to adding text before a mention. This was a regression from !158367 (diffs).

If you start the input by tagging ( @somebody) someone, and then relocate the cursor to before the @, you could type to insert text, but couldn't delete or otherwise interact with it (backspace and left arrow keys don't respond). When switching to plain-text, anything typed before the @ disappeared. This commit fixes the issue.

MR for master: !159685 (merged), Issue: #472988 (closed)

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• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
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