Implement RSS/Atom feeds

[JMAlego]

Implementation for RSS/Atom feeds for the website.

• RSS feed
• Atom feed
• Work out correct way to get absolute URL from flask, context.yaml possible?
• Validate generated XML as RSS/Atom

Fixes #159

[JMAlego]

@LukeMoll do you know the correct way to generate an absolute URL from flask? During development or when running dynamically you can use url_for with the _external=True parameter (which feels wrong as it is). When freezing we probably need to store the absolute URL in somewhere like context.yaml right?

[LukeMoll]

What feels wrong about _external? This seems like the most useful way to do it in Flask.

Given the GitHub repository is hard-coded in filters.py, I'd be comfortable hard-coding the website URL.

As per https://flask.palletsprojects.com/en/2.0.x/api/#flask.url_for, we can set the URL root with SERVER_NAME. The tricky bit would be handling the difference between development and production environments when freezing; the former probably wants it set to localhost:8000 (icr the port number), but the latter would use the actual webroot. We could set it to always use the actual webroot, but this could hinder development with absolute URLs (when looking at a locally-frozen copy). In fact, as long as this difference is documented, I think I'd be happy having the dynamic site use the default behaviour (from HTTP Host header) and all frozen sites use the website URL root (hard-coded or similar). Additionally, absolute URL use (that includes a host/domain, rather than being implicitly on the same domain) should be minimised.

The note in the docs that

Configuration values APPLICATION_ROOT and SERVER_NAME are only used when generating URLs outside of a request context.

is concerning, as AIUI we'd always be generating in a request context.

[JMAlego]

This is now ready for review with two open questions:

Where should we keep the files? I've gone with rss.xml and atom.xml but we could equally use feed.rss and feed.atom, there isn't really a standard.

Does all the information generated seem correct? It looks it to me and I've validated the generated RSS/Atom using the W3C validator (it is valid).

I've used FREEZER_BASE_URL which turns out to do exactly what I wanted, the default is hardcoded to https://hacksoc.org but that can be changed in production/development by Flask config.

Apart from that normal review things apply.

[LukeMoll]

Thanks for working on this, I'll try to give it a good look over on Monday. One quick comment based on the discussion here is that the cannonical URL for the website is www.HackSoc.org, so the base URL should probably reflect that. nginx is configured to redirect anyway so it shouldn't matter too much.

[LukeMoll]

Other than the minor documentation suggestion, looks good to me, thank you! Have tested RSS and Atom in Akregator and behave as expected.