Migrating AppProxies to different appIds

[sohkai]

Currently, AppProxies are assigned an appId that is assumed to never be changed.

While this has so far made it easy to validate an AppProxy's deployment and simplified various points of implementation, it has the drawback of making it impossible to migrate an AppProxy from using an app on one aragonPM registry to another.

A common usecase is a new app being published on an experimental registry, e.g. open.aragonpm.eth, and then later being improved and moved into the main registry, aragonpm.eth. Users of the original experimental app have no way, outside of uninstalling and reinstalling an app, to migrate to the improved version on the main registry.

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One potential implementation path is to modify both the Kernel and AragonApp:

• Kernel: expose a method, protected by APP_MANAGER_ROLE, that calls a method to change an appId on a given proxy
• AragonApp: expose a method, callable only by the connected kernel, that changes the internal appId