[PATCH v2] emacs: replace oldest-first with sort-order

[William Casarin]

Here's the before and after with from-ascending sorting on my rss tag:

before: https://jb55.com/s/e51db7073ccfc36d.png
after:  https://jb55.com/s/6a01cd3a0227ddfa.png

Last thing I need to add is a completing-read menu for selecting the
sort-order, since you can only set it in saved searches or with
notmuch-saved-search-sort-order at the moment.