MM-14579 Correctly size AutosizeTextarea on mount

[jwilander]

Summary

Depending on the order of events when the post/comment textboxes were being mounted, sometimes the height of the textbox would not be set due to races with when the post-create div was mounted. I removed the use of jquery to add a class name and instead used state in the CreatePost and CreateComment components to add the class as needed and made sure to call the size calculation on textbox mount.

Ticket Link

https://mattermost.atlassian.net/browse/MM-14579

Checklist

• Ran make check-style to check for style errors (required for all pull requests)
• Ran make test to ensure unit and component tests passed
• Added or updated unit tests (required for all new features)

[hmhealey]

Down with jquery magic!

Since onHeightChange is triggered on the AutosizeTextarea's component update and that triggers CreatePost and CreateComment to update, I'm a tiny bit worried about those two getting in a loop where they update each other. I don't think that can actually happen though since it would require some text that's long enough for the scrollbar to appear, and then when the scrollbar appears, the text height is short enough for the scrollbar to disappear again, but that shouldn't be possible since the scrollbar squishes the text horizontally and makes it taller.