Added rss_image to post

[g3rb3n]

Description

Adds functionality to include an image in a rss feed.

References

Adds #662

Checklist

• [X] I have read the contribution guide
• [X] Code lint checked via inv lint
• [X] changes file included (see docs)
• [ ] Usage documentation added in case of new features
• [X] Tests added

[yakky]

Thanks @g3rb3n for your contribution. I was wondering if you could the base post image as rss feed post image.

This will help reduce the complexity of the post admin

[yakky]

This has been merged by mistake with #729 , sorry

Thanks for your contribution, you will be credited for this :)