Deprecate content fields of article table

[robertu7]

Background

A follow-up task of article versioning. Keeping these legacy fields on article table could misguide our development.

Related Issues

• Article feed issues matters-web#3828
• Article resolvers return both Article and Draft type objects #2906

Checklist

• remove content fields: content, summary, etc.
• remove unused fields: upstream_id, etc.
• remove article.draft_id (since draft.article_id works the same);
• ipns-listen-repub is still relying on article table [0]

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Story

• Story: article versioning developer-resource#321

[guoliu]

article table act as pointers that point to different versions in draft table. article table record the newest version and metadata, while draft table record different version of content.