Highlight assigned engineers for on-call

Why is this change being made?

I am the on-call coordinator this month. As I was assigning shifts, I realized that it's going to be very difficult for me to differentiate between the shifts I've assigned and the shifts that've been volunteered for. I need to know that information, because the script that suggests engineers does not account for the engineers you've already assigned. I noticed in previous months that assigned names are highlighted in purple text, so I suggest we make this official to help others in the future.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
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