[EE] Fix failing E2E test ee_add_batch_comments_in_merge_request_spec.rb

What does this MR do?

Fixes the test by adding an include that was left out by an automated merge conflict.

From https://gitlab.com/gitlab-org/quality/nightly/issues/56#note_129658064:

What appears to have happened is that the CE changes from Collapsible Comments Test for Issues MR did not auto merge correctly onto EE. include Page::Component::Note did not get added to qa/qa/page/merge_request/show.rb and also the selector qa-reply-comment-button did not get merged causing Selector Sanity test failing

This also 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.

CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24247

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.
• 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