Executions are forked immedately on plugin start

[brollb]

When executing a pipeline, the execution is often created in a forked branch immediately.

I think this is because, when starting an execution plugin, the UI tries to record the execution ID of the plugin so it can be associated with the given execution across browser sessions, etc. However, as the plugin is also editing the model, this results in an immediate fork of the project. This would be able to be fixed nicely by having the plugin responsible for associating the execution of the plugin with the node in the model (blocked by webgme/webgme-engine#169).