Fix(oncallschedule): allow rotations to assign color

What does this MR do?

A small MVC for: #300181 (comment 495302550)

Generate color for each participant of the rotation (currently hardcoded to some shade of blue)

Where we allow each rotation participant to be assigned a possible color from the 30 combination color pallet.

Based on !52773 (merged) which is in review as well

How/What to test

🇺🇦 This feature is behind the feature flag oncall_schedules_mvc and the minimal license is GitLab Premium 👀

To check Enable the feature flag:

• Navigate to Operations => On-call Schedules or direct link is your_project/-/oncall_schedules e.g. http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/oncall_schedules.
• Click "Add schedule on empty state"
• Add a base schedule
• You will see a empty grid with no rotations
• Click add a rotation
• Add a rotation with a handful of members that have access to the current project and pick a start date / time
• A rotation will be added
• Each rotation will have a selection of assignee's drawn against the schedule/rotation data
• Each rotation assignee can be hovered over to confirm their start / end time
• These rotation items will dynamically resize depending on the screen size
• Verify that each rotation assignee is assigned a correct colour

This merge is part 2/4:

What	Merge
Update oncall Apollo cache methods	!52773 (merged)
Update oncall assignee colours	🤘
Add a base loader for grid changing	!52917 (merged)
Update mobile CSS and week calculations	!54003 (merged)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #262852 (closed)