Beforehand, visiting a permalink would cause Unddit to begin loading
from the Post's first comment. If the max-to-download setting was too
small, and the permalinked comment wasn't yet encountered, no comment
tree would be displayed.
Now, only comments that occur at the same time as the permalinked
comment or later (chronologically) are downloaded.
This also adds a Parent link to each comment. When viewing a permalinked
comment, clicking the top-level Parent link will download only new
comments between the parent and child comments. Likewise, clicking the
'view rest of comments' link will only download new comments from the
first comment up until the previously viewed comment.
In general, Unddit will not re-download comments that it knows it has
previously downloaded, nor download more than max-to-download comments
per link clicked. This can leave 'gaps' in the list of downloaded
comments. If the 'load more comments' link is clicked, Unddit will only
download comments that can fill such gaps between the currently-linked
permalink (or if not viewing one, the first comment) and the last, up to
the max-to-download setting.
* only rerender the comments after loading and iff something changes
* change text to 'loading...' and make link unclickable while waiting
* if all comments are already loaded, rename to 'load new comments'
