Make edit history available on help posts

[cellio]

Help topics can be edited by moderators, and when editing you can enter a revision comment (like for regular posts). This implies that a history is kept in the database, but there's no way to see it through the UI. This request is to expose edit history for help topics. The history should be visible to anybody who can see the topic -- usually everybody, but there are a few mod-only topics.

[cellio]

If the information is in the database then this should be an easy UI change (with an added access check). If the information isn't already being stored, then this becomes a much bigger request that should be re-evaluated for priority.

[cellio]

The edit history will also need to account for changed made via UPDATE_POSTS=true rails db:seed. I don't know if seed updates record their changes anywhere now.

[trichoplax]

If it turned out that the history was not being recorded, would that make this issue high priority? If so, is it worth labelling it high priority until that is determined, and then downgrading it to medium priority if it turns out the history is already stored?

[cellio]

I had a help topic get clobbered a few months ago, so I think it's not recorded in the DB. Raising the priority.

[Oaphi]

I had a help topic get clobbered a few months ago, so I think it's not recorded in the DB. Raising the priority.

@cellio we had a bug that I fixed in #1201 - history for help / policy posts is recorded, it's just that a bug resulted in content being clobbered every time seeds with UPDATE_POSTS set to true have been run.