Splits auto-refreshing of MR widget into 2 requests:
- the one which uses etag-caching and invalidates the fields on change
- the one without caching
The idea is to gradually move all the fields to etag-cached endpoint
This commits extracts /merge_requests/1.json?serializer=widget
Into a separate /merge_requests/1/widget.json endpoint
This will allow to use caching for this request