feat(scheduled reports): Add Schedule Report modal & Explore saved query integration

[shashjar]

https://www.notion.so/sentry/Scheduled-Reports-on-Saved-Queries-Dashboards-3098b10e4b5d8017808bdea381cb4465

Follow-up to #108418

PR 5 for adding scheduled report delivery for Explore saved queries (CSV email attachments) and eventually Dashboards (likely PDF email attachments).

• Adds the ScheduleReportModal component for creating scheduled reports on Explore saved queries
• Adds a "Schedule Report" menu item to the saved query edit dropdown, gated behind the scheduled-reports feature flag

NOTE that the frontend implementation is not complete:

• The modal does not look/feel very nice right now. Need to improve the design & UX, and more thoroughly test it e2e.
• No visibility into existing schedules. After creating a report, clicking "Schedule Report" again creates a blank create modal with no indication that a schedule already exists.
• No edit mode. There should be a way to edit (modify configuration, delete, disable, enable) a given scheduled report.
• No visual indicator on the saved query page to signal that a query has active scheduled reports.
• We may want a separate frontend somewhere to display all scheduled reports configured within an organization.