Remove non-user links from todos and highlight current user

What does this MR do and why?

Previously we had links show up in the preview text of To-Dos. This change makes them pure content as they (a) either were too distracting or (b) were not actually discoverable and violated the Pajamas guidelines.

We also fixed the highlighting for the current user if they are mentioned in a To-Do.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Create a To-Do for yourself where the preview text would include a link.

MR acceptance checklist

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