Add bulk update overrides modal

What does this MR do and why?

Bootstraps the modal component for bulk caps addition. No mechanics for now, except the show/close/clear actions.

Implementation status

Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/630981

Piece	Work item	MR	Status
Feature flag credit_caps_bulk_update + scaffold button	https://gitlab.com/gitlab-org/gitlab/-/work_items/630982	!257651 (merged)	✅ Merged
BE: username on BudgetCapUserOverrideInput	https://gitlab.com/gitlab-org/gitlab/-/work_items/630983	!258407 (merged)	✅ Merged
FE: bulk update overrides modal	https://gitlab.com/gitlab-org/gitlab/-/work_items/630982	!257948 (merged)	⬅️ Current
FE: CSV parser	https://gitlab.com/gitlab-org/gitlab/-/work_items/630984	!258244 (merged)
FE: integrate modal with BE	https://gitlab.com/gitlab-org/gitlab/-/work_items/630985	!258465 (merged)
FE: CSV parser improvements (follow-up)	https://gitlab.com/gitlab-org/gitlab/-/work_items/631895	!259381 (merged)
Documentation update	https://gitlab.com/gitlab-org/gitlab/-/work_items/632125	!259654 (merged)
Feature flag rollout	https://gitlab.com/gitlab-org/gitlab/-/work_items/631018

References

• Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/630982

• Depends on: !257651 (merged)

Screenshots or screen recordings

Before	After
N/A

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:credit_caps_bulk_update)

2. Visit admin area

3. Then GitLab Credits page

4. there click on Credit caps, leading to http://localhost:3000/admin/gitlab_credits_dashboard/credit_caps

5. Click on Bulk update button

6. Interact with the modal (fill text area, cancel, save)

7. There's no functionality except open/close/clear on the modal (that's expected)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.