Massively simplify everything

Some highlights:

- use functions instead of bloated classes
- don't futz around with uploading templates
- frontend is no longer uploaded
- the code handling title-less feeds is easier to follow
- change how we track stuff in redis
populate_feed_object: handle feeds w/ <title> but no <description>

These feeds technically break the RSS spec, but it's an easy fix.
clean_text: use &nbsp; with ellipsis
generate_manifest: use relative URLs for river.js files
Update README
Delete README.md
Update README.org intro
Add link to web frontend
Fix borked web frontend link
Update README.org
Tweak README.org
s/TODO/Unfinished/
Merge branch 'riverpy2'