Use avatar subdirs

[janboddez]

Like, not month and year, but just the two first characters of the filename (hash). Like how caching solutions do it.

Month and year (which IndieBlocks does not use, but https://github.com/janboddez/addon-for-activitypub does) aren't great, because they cause file paths to change every month, and therefore unnecessary duplicates and so on. :-)