Option to set default avatar by class.

[toonarmycaptain]

Might be nice for user to be able to set a default avatar for each class, rather than a 'dionysus_default' avatar.
This shouldn't be too hard to implement, have a 'classname_default_avatar.png' in the class avatars folder, and a value in the class_data_file indicating a default avatar is provided. Or simply save as __classname_default_avatar, and make sure any user student names have prefixed underscores stripped or otherwise prevent user ending up with an avatar with that reserved name, then check for that name in avatars folder when default_avatar.png is the student's avatar, or/and change each student without a provided avatar to that name when a custom default avatar is provided.

[toonarmycaptain]

GitMate.io thinks possibly related issues are #69 (Use default avatar in place of bad avatar), #33 (Take avatar images and process them), #81 (Add tests for avatar copy funct), #92 (Decouple class functions from UI), and #14 (Settle on class data format).