feat: Add a highlight to messages containing mentions of the current user

[Sollace]

…or room

Description

Much like how discord does it, this just makes messages where you're mentioned stand out better which will make them easier to spot when scrolling.

The highlight is also useful to know if a mention you sent to the room worked or not, as new users may assume things like @here works like they do on discord and may not realise until much later that may have notified only one (or zero) users.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

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