Properly disable buttons on submit

When comments are submitted, the "Comment" button changes to "Commenting…" but isn't properly disabled (I believe). This was basically a hack, but we should actually disable the button, and remove the Cancel button.

This may require a little bit of refactoring; I think the hack existed because there was no proper way to disable the button.

I think the same may also apply to the post form, but I'm not certain.

We should implement the following:
* [ ] Disable or remove the cancel button when a comment is being submitted
* [ ] Disable the comment button when a comment is being submitted, or replace the whole section with text instead of buttons
* [ ] Check post submission and apply the same if needed