Fix add_batch_comments_in_merge_request spec on staging

What does this MR do and why?

Describe in detail what your merge request does and why.

E2E test add_batch_comments_in_merge_request_spec.rb is failing on staging as test environment broadcast banner is covering Submit button (see #364073 (closed)). This MR modifies the flow so that we click Submit button in the comment which is synchronised with the footer and has exactly the same effect.

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How to set up and validate locally

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bundle exec bin/qa Test::Instance::All http://localhost:3000/ -- qa/specs/features/ee/browser_ui/3_create/merge_request/add_batch_comments_in_merge_request_spec.rb

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