[CE] port of fix for failing E2E test ee_add_batch_comments_in_merge_request_spec.rb

What does this MR do?

CE changes from EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9084

Splits the reply_to_discussion method.

It has 2 parts, entering text and submitting the comment. They have to be separable because the EE batch comments test performs different actions after entering text - it doesn't always immediately submit the comment.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/quality/nightly/issues/56

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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.
• Security reports checked/validated by reviewer