Remove Avatar from other models
Remove Avatar from admin.py
Remove Avatar from fixtures
Remove Avatar model
Add migration to remove Avatar
Remove unused pre_delete signal receiver
Create thumbnails on post_save
Change thumbnail name format
Rename AVATAR_THUMBNAIL_SIZE setting to COMPETITION_THUMBNAIL_SIZE
Refactor create_thumbnail
Remove avatar from templates
Add a setting for default competition images
Show default competition image if there isn't one set
Merge branch 'feature/remove-avatar' into develop