api: minor fixes to make API more uniform
api: minor fixes to make API more uniform

The most important change is to make user_groups field of Group object
read_only. This prevents from accidentally removing all user groups when
making update. Other changes make the serializers more uniform across
all API.

Signed-off-by: Milosz Wasilewski <a39812e55748be210556797addcf4c064734f1bf@linaro.org>

terceiro/squad,terceiro/squad,terceiro/squad,terceiro/squad