Expose config change handler to Plugin, instead of deleting and re-creating

[gigxz]

When the configuration of an active plugin changes, metagov-core deletes the plugin instance and creates a brand new one. This means that all the plugin state is lost. This might not be desirable for some plugins that use state to persist data that is updated via actions (see the "revshare" plugin).

Instead of deleting and recreating plugins, expose a handler that lets the plugin author decide what should happen when the config is changed.
Related to discussion #16.

Relevant code in core:
https://github.com/metagov/metagov-prototype/blob/70e8b4752e82b1bfc11dd3274071acd5734ce94c/metagov/metagov/core/serializers.py#L30-L34