Use shared Redux store code across background, panel, and full-tab UI

[lmorchard]

TL;DR: The basic setup could work something like this:

• Event listeners on the Login API dispatch actions to the background Redux store.
• UI views request initial local Redux state from background script on init.
• Middleware installed in the background store relays actions to message ports on open UI views.
• Relayed actions are dispatched in local UI stores to maintain sync.

More background:

The addon is composed of a background script, a panel UI, and a full-tab management UI. Currently, each of these pieces has its own data management code - the background has a datastore.js, and the two UI views have variant Redux stores.

A single Redux store module could be shared between all parts of the add-on. There, we could define one common set of actions and a data store modeled after the Logins API (ala #21). Redux actions map well to messages, which also reduces complexity between background and UI views.

[lmorchard]

This seems like a neat, low-priority idea for enhancement. Filing an issue to solicit poking holes in it. A few to start:

• If it really is a single Redux store implementation, there will need to be UI state related data in it that's not directly related to Logins API data. (e.g. current selected item in full-tab management view)
• Are there performance or memory concerns in a Redux store? (I'm thinking not significant concerns, but I could be mistaken)