Merge branch 'add_feed_id_to_subscribeToFeed_response' into 'master'

Add feed id to subscribe to feed response

As described on the forum, this proposed change is to make the subscribeToFeed API response include the feed_id of the new or existing feed.

https://tt-rss.org/forum/viewtopic.php?f=8&t=3893

Thank you.

John

See merge request !35