Create and edit relation:enforcement

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I understand that working with relations is something that is hard to implement on the mobile editor. At the same time #328 mentioned that it could be done by providing a whitelist of specific relations and roles.

In my opinion, an enforcement relation is a perfect candidate for this approach. According to osm wiki examples and taginfo, it's feasible to support a limited scope of ways and nodes for a small set of roles.

https://wiki.openstreetmap.org/wiki/Relation%3Aenforcement

I believe it's possible to provide a simple UX for an enforcement relation so that it's easy to create and edit while impossible to break things. If that is achieved, #486 could be done in the same way.

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On the other hand, maybe turn restrictions would be easier to implement first, since they are already displayed.