Update oncall grid UX

What does this MR do?

A small MVC for a set of issues:

We seem to have an issue with the placement of shifts when a rotation is created which starts in the past. The red cursor doesn't actually overlap the current shift

Another related issue that I can raise separately if that's easier - when you edit the timezone of a schedule, it updates the top of the card body, but doesn't update the timeline or tooltips.

To handle this we:

• Update the weekly grid current time indication to track hours as well as days i.e. it moves through the day based on the user's time.
• Give the smaller elements some love by hiding the rotation assignee avatar for smaller shift sizes allowing for information to be viewed.
• Allow the page to reload when a user changes a schedule timezone.

Note

This merge also contains changes from:

• Feat: allow grid to start at week beginning
• Fix: oncall visual updates

These merges were reviewed and approved separately to this merge.

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 an 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 the rotation start/end times for the week view

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this feature sits behind a feature flag .
• [-] Documentation (if required)
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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 #322137