Added Generate Excerpt for the Classic Editor.

[jamesmorrison]

Description of the Change

Adds the Excerpt Generation (introduced in #405 for Gutenberg) to the classic editor.

Closes #417

How to test the Change

1. Create a post using the classic editor
2. Add some content and save as a draft (note: a draft must be saved as we need a post ID to make the OpenAI request)
3. Locate the excerpt box and click "Generate excerpt"
4. Button should be disabled; content should be added to the excerpt textarea; button text is updated (to Re-generate excerpt); button should be enabled.

Changelog Entry

Added - Excerpt Generation support in the classic editor.

Credits

Props @jamesmorrison, @ravinderk, @dkotter

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[ravinderk]

@jamesmorrison, Can you resolve code conflicts?

[ravinderk]

@dkotter This pull request is ready for review.