Article versioning, extended units and alternative article sharing (#1058)

[lentschi]

In this PR (addressing #1058) we originally wanted to extend article units only, however during development it became clear, that other parts had to be refactored/rewritten too - so it now encompasses these three major changes:

1. Article versioning
2. Article list synchronization
3. Article unit system

For details see the architecture notes describing the changes.

[twothreenine]

Solves #422 Improve unit handling
Solves #474 by allowing price input per price unit (I think the rounding discrepancy should be in a tolerable range now)
Solves #827 Minimum order quantity for certain articles
Solves #843 Allow to insert an integer / number in orders to set amount of a certain product
Solves #850: Full article versioning (still to implement: apply new version specifically to future/current/closed/settled orders, and/or specific order)
Fixes #1056 Critical bug: Article price changes when closing order
Makes #710 obsolete (unit_quantity == 0 causes 500 when viewing group_order)

We've decided against supporting ordering partial cases, so #645 won't be solved, however if you choose a scalar unit, you can order loose quantities like 2.5 kg.

Sharedlist issues which will probably be affected:
#675 feature: (Un)check articles for article sync
#1030 Synchronize from external database fails with "out of range" error
#1033 Access control for shared suppliers

Note: Implementation of #569 (Work with datafoodconsortium) could be done based on this PR.

to-do: Check issue numbers below 400