Add Rspec modal helpers

What does this MR do and why?

Adds two Rspec helpers for interacting with GitLab UI modals in feature specs.

• within_modal is a general helper for interacting with GitLab UI modals
• accept_gl_confirm will be helpful for &6993 (closed) and can be used to replace accept_confirm

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.