Added include_in_rss field to Post

[g3rb3n]

Description

Adds functionality to exclude certain posts from the rss feed.

References

Adds #663

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 your contribution @g3rb3n , I merged this with #728 to ease porting to updated develop branch