This is step 3 of the comment serialization cleanup. The way that diff

comments are serialized/deserialized is very different from the way that

file attachment comments are handled. For file attachment comments (and

screenshot comments), we automatically pull out several keys from the

first comment in each block's serialized comment array using the block

model's serializedFields list.
Diff comments currently use a very different format, and are

deserialized differently. A later change will update the diff comments

to use the same serialization style as attachment/screenshot comments.

This change prepares for that by moving the serializedFields list down

from FileAttachmentCommentBlock into AbstractCommentBlock.

Ran js-tests.