use webp for avatars and convert current avatars, fixes #3183

.gitignore

app/controllers/avatar.php

app/controllers/privacy.php

app/routes/User.php

app/views/admin/licenses/edit.php

app/views/avatar/update.php

db/migrations/5.5.10_convert_avatars_to_webp.php

lib/bootstrap.php

lib/classes/Assets.class.php

lib/classes/Avatar.class.php

lib/classes/CourseAvatar.class.php

lib/classes/InstituteAvatar.class.php

lib/classes/JsonApi/Schemas/User.php

lib/classes/LicenseAvatar.php

lib/classes/StudygroupAvatar.class.php

lib/models/User.class.php

lib/phplib/Seminar_Auth.class.php

public/assets/images/avatars/course/nobody_medium.webp

public/assets/images/avatars/course/nobody_normal.webp

public/assets/images/avatars/course/nobody_small.webp