Add rotation to the schedule view via single token

What does this MR do?

A tiny MVC for #262860 (closed) where we allow the schedule grid positions to be calculated and drawn dynamically and supporting specs for which are currently hooked up to mock data i.e. mocks/mock_rotation.json. To begin with we are starting with days only.

🇺🇦 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 new grid pre-populated with a set of fake rotations
• 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
• To verify the calculations you can either edit the startsAt / endsAt value for any shift participant inside ee/spec/frontend/oncall_schedule/mocks/mock_rotation.json or update the initialDate param inside ee/app/assets/javascripts/oncall_schedules/components/schedule/utils.js. This will let you see different variations for timeframe items and shifts.

This merge is part 2/4:

What	Merge
Add base rotation / schedule grid	!51330 (merged)
Allow overlapping rotations be contain a single name / avatar instead of multiple(UX review happens here)	🤘
Replace fake data with @client GraphQL	TBA
Replace fake data with actual backend code	TBA

Screenshots (strongly suggested)

LG	MD	SM	XS

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 #262860 (closed)