On #736: Added cleanup migrations

[lentschi]

(squashed and modified commits from issue_736)

[paroga]

THX, first check LGTM. i'll give it a deep check in the next days and try it on some database to check if there are issues

[lentschi]

I just fixed the unit tests by creating temporary tables for shared_articles and shared_suppliers. They aren't real temporary tables, but that I couldn't get to work with ActiveRecord for some reason.
I still believe it's cleaner than the current way of 'recycling' the internal articles and suppliers tables for testing though. (They have a completely different structure after all.)

The cleanest overall solution (apart from test refactoring) would be to use an API for this kind of import/export (or at least some sort of shared schema via a gem), but that's outside of the scope of this PR.

[RayOei]

What is the intent here as is still in Draft and has been for some time 😇
Added clear link to #736 for navigation.

[lentschi]

What is the intent here as is still in Draft and has been for some time

I'm not sure, what you're asking...

• "Why is it still in Draft and not being merged?" -> Because we didn't dare to merge it back then as we didn't have the ressources to review it thoroughly and it might lead to the loss of data. (Actually I later found a few of the foreign key constraints I made here are simply wrong, but I didn't want to push this through on my own, so I hadn't invested time in correcting them.)
• "Why is it still in Draft and not Closed?" -> Because the code might still be useful. Now that we have a bigger team, we might have another go at it. (But after Article versioning, extended units and alternative article sharing (#1058) #1073 we'll have to rewrite a lot of it, of course.)