proposal: rules should be ordered by specifying which rule should come before it

[bhearsum]

Rather than setting a priority of a rule to determine where it sits in order, it would be neat if you could say which rule should come before it and have Balrog internally decide the priority based on that.

This would be similar to a linked list solution where you can insert based on appending at an indexed value and all nodes in the list would be re-ordered based on that.

Put another way, the priority wouldn't be an exposed for mutability.

Motivation:

Often as releaseduty, we have to re-order the priority of many rules to fit others in. This opens us up to a silly human mistake if you are scheduling many rule changes and also more churn than needed as both releng and relman must sign off on a no-op change.

This would require front end work too

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1426218 by @lundjordan)