Add more information into RSS feed for issues

## What does this MR do?

This MR adds issue text, labels , milestone, assignee and due date into issues RSS feed.

## Are there points in the code the reviewer needs to double check?

#14572 requests to add 'weight' among other fields. Seems like issue weight is available
in enterprise edition only so it is not implemented in this MR. Please correct me if I'm wrong.

## Why was this MR needed?

This MR is needed because it extends issues RSS feed with useful information requested in
#14572.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/14572

See merge request !4158