Warn about impacted escalation policies when members are deleted/leave

What does this MR do?

Related issue: #335546 (closed)

This MR updates the warning modals a user will see when trying to leave a group/project or delete a user. Escalation policies route alerts to either on-call schedules or directly to users. On-call schedules are already listed in the modal, so this MR adds a list of escalation policies which route alerts directly to users.

Changes

Backend:

• Adds support for escalation policies to the relevant internal APIs backend
• Limits listed on-call schedules to only the ones in which the user is an active participant
• De-dupes on-call schedules for a user in the group/project member list

Frontend:

• Renames OncallSchedulesList to UserDeletionObstaclesList & all corresponding props/files frontend
• Adds escalation policies to UserDeletionObstaclesList

Screenshots or Screencasts

User removing self [leave modal]	Owner removing user [remove member modal]	Admin deleting [delete user modal]	Admin deleting with contributions [delete user modal]

Out of scope (will be other MRs)

• Deleting the escalation rules corresponding to the user
• Sending an email to project/group owners informing them of the change

How to setup and validate locally

1. Removing yourself from a group/project
   • Navigate to Project information/Group information > Members > find self > click
2. Remove a user from a group/project (as owner/maintainer)
   • Navigate to Project information/Group information > Members > find user > click
3. Deleting user from instance (as admin)
   • Navigate to Menu > Admin > Overview > Users > find user > click > Delete user/Delete user and contributions

Streamlined testing flow:

• Create a new account & logout (skip if you have a non-admin user already)
• Sign in as instance administrator
• Create a project
• Add new user as a developer on the project
• Setup on-call schedules & escalation policies which direct alerts to developer
• Navigate to members list to see removal modal
• Navigate to instance user list to see deletion modal
• Logout & Login as developer
• Navigate to members list to see removal modal

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
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• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• [🤞] This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.