This commit reformats hashed medium static avatar files("-medium.png"
files). Unlike user-uploaded avatar files, avatars served alongside
static files are hashed by Django. To implement the URL patterns for
finding medium-size avatar URLs that are hardcoded into current
versions of the mobile apps, the medium avatar files need to be
adjusted to include the "-medium" string just before the file type.
For example, the URL format will now be:
Before: welcome-bot-medium.123123321.png
After: welcome-bot.123123123-medium.png
433968cbb6