Previously, requesting system bots URLs did not return any -medium.png
variants and SVG file was also used for notification bots' avatar, which
was problematic.
In this commit, the -medium.png variants is added for the avatars of
system bots and zulip-icon-square.svg is also converted into
notification-bot.png for the notification bot. The get_avatar_url method
has been updated to return the "medium" file variants for the system
bots.
Additionally, the system bots' avatar files is moved to a dedicated
directory to simplify the hashing logic for these files. Now, all files
in the "images/static_avatars/" directory will be hashed.
PieterCK