Move styles from SubmissionComment to MarkdownTextarea

[matthew-white]

I was randomly looking at SubmissionComment and noticed that it sets styles on #submission-comment-actions. However, it looks like there isn't a #submission-comment-actions element anymore: I think that element was moved into MarkdownTextarea as part of #483 more than a year ago. This PR moves the styles from SubmissionComment into MarkdownTextarea, though given that they haven't been in use for a while, we'd probably also be fine removing them.

The styles do two things:

• Adds 5px of margin-top.
• Adds an animation on margin-bottom.

I've recorded the current behavior (without these styes) and the behavior with these styles. I slowed down the animation in the recording so that it's easier to see.

Current behavior (without styles):

Screen.Recording.2023-04-10.at.2.38.18.PM.mov

With styles:

Screen.Recording.2023-04-10.at.2.33.36.PM.mov

@ktuite, do you remember these styles at all? Do you know whether we intended to remove them?

Before submitting this PR, please make sure you have:

• run npm run test and npm run lint and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced