Summary:
*Stack summary*: this stack refactors plugin management actions to perform them in a dispatcher rather than in the root reducer (store.tsx) as all of these actions has side effects. To do that, we store requested plugin management actions (install/update/uninstall, star/unstar) in a queue which is then handled by pluginManager dispatcher. This dispatcher then dispatches all required state updates.
*Diff summary*: refactored Flipper mocking helpers to allow testing of plugin commands, and wrote some tests for pluginManager.
Reviewed By: mweststrate
Differential Revision: D26450344
fbshipit-source-id: 0e8414517cc1ad353781dffd7ffb4a5f9a815d38
