fixup! Cleanup, refactor and test new AvatarService

js/models/messagecollection.js

js/views/messagesitem.js

lib/Contracts/IAvatarService.php

lib/Controller/AvatarsController.php

lib/Http/AvatarDownloadResponse.php

lib/Http/CacheHeaders.php

lib/Http/JSONResponse.php

lib/Service/Avatar/AddressbookSource.php

lib/Service/Avatar/Avatar.php

lib/Service/Avatar/AvatarFactory.php

lib/Service/Avatar/Cache.php

lib/Service/Avatar/CompositeAvatarSource.php

lib/Service/Avatar/FaviconSource.php

lib/Service/Avatar/GravatarSource.php

lib/Service/Avatar/IAvatarSource.php

lib/Service/AvatarService.php

tests/Controller/AvatarControllerTest.php

tests/Http/AvatarDownloadResponseTest.php

tests/Integration/Service/AvatarServiceIntegrationTest.php

tests/Service/Avatar/AddressbookSourceTest.php