Dependent logs

Problem/Motivation

In some case, you may want to make one log dependent on another (or multiple) logs that come before it.

For example, in grazing rotations, you shouldn't mark rotation #2 done before you mark rotation #1 done (see https://github.com/farmOS/farm_grazing/issues/28).

Proposed resolution

This could be a simple field or property on log entities that points to one or more other logs, and would not allow you to mark a log "done" until all its dependencies are.

Remaining tasks

Figure out:

- Do we need to also enforce that dependency logs always have a timestamp BEFORE the dependent logs (that seems like a requirement, conceptually - but maybe isn't really necessary to enforce? It is certainly a bit more complex) - if a log's timestamp changes we would need to enforce that it doesn't end up out of order, and therefore there would need to be an accepted way to reorder logs if necessary
- What does the UI look like for adding dependencies to a log? Just a log reference field?
- Where does all the logic need to be for enforcing? Insert/update/delete hooks? Use the new farm_constraint system?
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User interface changes

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API changes

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Data model changes

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