Let lock managers edit checkout URL

[julien51]

Right now, we use the createdBy field on a checkout URL to list them and/or enable edits.
Instead of relying on this mechanism for "access control" we should use lock managers.

[julien51]

We will likely need to solve this in 2 steps:

1. Let a lock manager "save" a config for which they are not the creator, but for which they are lock manager on one of the locks. This will first require changes in locksmith, and then changes in the UI
2. List configs that were not created by a user, but for which the user is a lock manager on one of the locks.

The checkout URL builder already supports passing a checkout config id in its URL like https://app.unlock-protocol.com/locks/checkout-url?id=3f2ee0bd-077f-4c8b-bd11-85038692006e . this will make it easier to work on the 1st step.
If the 2nd step is too complicated (listing configs for which one of the lock's is managed by the current user) is going to be wasteful, then we can take an intermediate of allowing the user to enter a custom checkout id in the step to select an existing checkout id.

[0xTxbi]

got it!