Resolve "Unbox on-call schedule UI" - rotations

What does this MR do and why?

1. changing the background to grey
2. removing the border outside the timeline and within the dates indicators
3. changing date format, font colours and weight for date indicators
4. adding separator between the weeks
5. rounding the assignees indicators on timeline and aligning their avatar + name to the left

Screenshots or screen recordings

before	after

How to set up and validate locally

Prerequisite:

You must have at least the Maintainer role.

To create an on-call schedule:

• On the top bar, select Main menu > Projects and find your project.
• On the left sidebar, select Monitor > On-call Schedules.
• Select Add a schedule.
• Enter the schedule’s name and description and select a time zone.
• Select Add schedule.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #387079 (closed)