Handle no avatar, add delete avatar support

frontend/src/api.js

@@ -100,6 +100,7 @@ export const api = {
     form.append('avatar', file)
     return request('PUT', '/api/profile/avatar', form)
   },
+  deleteAvatar: () => request('DELETE', '/api/profile/avatar'),
   avatarUrl: () => '/api/profile/avatar',
 
   // sessions