Improve comments functionality

[malachig]

In adding some comments to a variant page in CIViC v2 I noticed several limitations compared to what we used to be able to do in CIViC v1.

https://civicdb.org/variants/4415/comments

• [x] Markdown is no longer rendered
• [x] The "Preview" view doesn't seem to do anything.
• [x] You can no longer expand the size of the comment window. For complex comments this makes it hard to check for errors/consistency.
• [x] A user can no longer delete their own comment. If you make any mistakes, all you can do is leave another comment.
• [x] The comments page in the admin interface gives an error: HTTP ERROR 500. So you can't delete a messy comment there either.

[acoffman]

All of these points are addressed in #1108 and will be fixed when it is deployed!