Fix(oncallschedule): add base loader

What does this MR do?

A small MVC for:

Restore empty grid view - currently, we have only a header when there is no rotations, but at some point, there was an empty grid

Where we add a empty grid for the current shift type(days/weeks), add a loader for new grid requests and remove the add/update cache methods for the rotations as we need to request these from the backend each time as the SSoT for shifts.

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 there is an empty grid and you can navigate between weeks / days with a loader

This merge is part 3/4:

What	Merge
Update oncall Apollo cache methods	!52773 (merged)
Update oncall assignee colours	!52839 (merged)
Add a base loader for grid changing	🤘
Update mobile CSS and week calculations	!54003 (merged)

Screenshots (strongly suggested)

R1

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)