Extract list_projects into a separate controller

What does this MR do?

This a part of a series of refactorings to extract parts out of ErrorTrackingController towards more RESTful controllers.

Extracts ErrorTrackingController#list_projects to ErrorTracking::ProjectsController

The main goal of the change is moving towards RESTful controllers, and have a separate controller per resource. As well as unload the ErrorTrackingController by extracting more code out of it.

It also follows an established practice to use the HTTP GET method for reading data.

• Extract Sentry project resource into a separate controller
• Change HTTP method from POST to GET
• Update front-end part of using a new route

Screenshots

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

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