1st steps towards avatar uploading
moving functions and test to where they belong (avatarProvider)
continuing to provide a nice file-upload experience
linting and unit testing
detecting files as images via mime-types
avatars now saved like other images (by uuid)
moved avatarUrl creation to avatarProvider
finalizing edit avatar page
Merge pull request #865 from NicoleRauch/master

First steps toward displaying member information in SoCraTes.
Merge branch 'master' into profile-pictures
updated layout for editing avatars and refactorings
documentation update for avatar images
tweaking UI (hiding text in "normal" delete buttons)
Merge pull request #866 from leider/master

Now avatars can be customized