Add custom notification settings modal

What does this MR do?

In !52068 (merged) we introduced the notification settings dropdown Vue component. This MR adds another component that interacts with the dropdown: the custom notification settings modal which is shown when the user wants to edit custom notification settings for a project, group or globally in the user settings.

This is behind the vue_notification_dropdown feature flag.

Screenshots (strongly suggested)

Project	Group	Notification Settings
custom_settings_modal_project	custom_settings_modal_group	custom_settings_modal_user_settings

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