[Starts #42027463] Adds an avatar model
Refactors deprecated simple_form method
Updates rails to patch security
Ignore public uploads
Add avatar to groups
Adds a bunch of fixture files to spec/support
Adds :small, :medium and :large avatar sizes
Seed and test avatars default to fixture files
Improves styling of avatar selection
Fixes the failing test

This was a bug in the test, that tested for the wrong size of the image
in the dom.
Removes left-over focus tag
[Completes #42027463] Tweaks group list item layout
Merge pull request #19 from ekampp/features/group_avatar

[Delivers #42027463] Enables selecting an avatar, when creating a group.