Resolve "Add own avatar support"

Closes #68 (closed)

• Remove gravatar integration completely
• Add default image
• Determine all necessary sizes (so far: [320, 192, 160, 80, 40]
• Allow admins to change profile images of other users
• Enhance error handling and user feedback for it
• Add UI for opening upload modal in user-form
• Add Option to delete profile image
• Fix cropped area might overflow image boundaries
• Test on mobile
• Fix tests