Indicate unsupported versions in RSS feed
Use a proper database field for beta releases

This replaces the previous ugly hack where beta versions had a negative number
as latestminor, giving a much cleaner model.

Requires SQL:
ALTER TABLE core_version ADD COLUMN beta boolean NOT NULL DEFAULT 'f';
ALTER TABLE core_version ALTER COLUMN beta DROP DEFAULT;

(Yes, we really need to move to something that can do automatic migrations)
Explicitly list beta and development versions in docs as "devel"

Previously we listed both those as unsupported, which confused some users
particularly with new beta releases. While they are technically not supported
yet, it gave off the image that specific features would not be in newer
versions anymore, since they got listed as unsupported.

In passing, fix the style of the links in case they only exist in old
versions, which would put an extra | character in the version list.