ui/services: sort on call users by escalation step

[KatieMSB]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
Updates the Service Details On Call Users list to have collapsable sections for each step for on call users.

Which issue(s) this PR fixes:
Fixes #2126

Screenshots:

[mastercactapus]

Looks good!

One thing of note we didn't consider: if there are no users on a step, it's omitted entirely -- probably okay, but I could also see users raising questions, "hey, why isn't step 2 showing up?"

We could pull in the escalation policy to display all steps, though we should probably add # of on-call users in each section as subtext or something for that as a hint.

Or maybe it's all the better as a follow-up if someone requests it.

@Forfold Thoughts?

[Forfold]

Yeah that sounds like a good idea. Display the step if even if there's nobody on it. The accordion details could just have some typography like "Nobody on-call for this step" and there could be a warning icon rendered to the left of the Step # text in the accordion summary

[johnmarksilly]

Changes look good!