Add Sentry project dropdown to error tracking settings form

What does this MR do?

Last release we had added an integration with Sentry where the user can add his Sentry API URL (ex: http://sentry.example.com/api/0/projects/organization-slug/project-slug/) and token and we display a list of Sentry issues/errors.

This issue is to make it easier for the user to set their Sentry API URL. In this issue, we aim to add a project selection dropdown, so that the user only has to enter his Sentry API host (ex: http://sentry.example.com) and token, then we populate the dropdown with a list of Sentry Organizations and Sentry Projects that the user has access to, and they can choose the project whose issues they want us to display.

This MR is the EE counterpart to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24701.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/55199

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer