Add frontend for instance-level integration overrides

What does this MR do?

This MR adds a listing of projects using custom settings (what we refer to as "overrides") for a specific integration. It's behind the feature flag instance_level_integration_overrides (#336750 (closed)).

• Fetch overrides from the backend along with pagination info.
• Add table with overrides that shows the project avatar, name and links to project root path. In future iterations, this will include an Actions column that will allow admins to reset such project integrations.
• Pagination for overrides that only shows when there are more than 20 records.

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

1. Enable the feature

   Feature.enable(:instance_level_integration_overrides)

2. Visit a project's Campfire integration setting page such as http://127.0.0.1:3000/flightjs/flight/-/services/campfire/edit.
3. Enter anything into the "Campfire token" field and save.
4. Visit the instance admin's Campfire integration setting page http://127.0.0.1:3000/admin/application_settings/integrations/campfire/edit.
5. Click the "Projects using custom settings" tab.
6. The project should appear in the list.
7. Click the "Settings" tab now.
8. Enter a different "Campfire token" into the field and save. A modal will appear. Click save.
9. Visit the same project's Campfire integration setting page again such as http://127.0.0.1:3000/flightjs/flight/-/services/campfire/edit.
10. Change the drop-down from "Use custom settings" to "Use default settings".
11. Click save.
12. Visit the instance admin's "Projects using custom settings" page again http://127.0.0.1:3000/admin/application_settings/integrations/campfire/overrides.
13. The project should no longer appear in the list.

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?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] 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.

Related to #218252 (closed)