Early-stage JS messaging refactoring

[jamierumbelow]

This PR starts to standardise the messaging format between the different JS files, with the goal of being able to validate and type messages from cradle to grave, from JS to Swift and back again.

The core idea is that all JS files communicate with the following object:

type Message = { destination: "background" | "content" | "popup";  method: string; params: any; sessionId: string; }

destination is used to multiplex the browser runtime events channel, and permit more generic method names. method is currently general-purpose, but will later be typed, ditto params. sessionId is currently unused.

It also:

• simplifies the view rendering logic
• removes much of the global state from background, content and popup, preferring a more injected, localised and functional approach
• adds tab-specific sessionId generation, which is stored in and read from localStorage
• adds a lot of logging to the frontend code
• adds a very rough loading state for initial popup load

My approach here has been to try to build in an 'invisible' way, so the extension should work mostly as it did before. Signing wasn't working for me before, so I've been unable to test it properly.

Explicitly not included in this PR:

• TypeScript
• Reusability of messaging primatives between JS files (ie there is some duplication right now)
• Unit testing of messaging primatives
• Standardisation of the JS <-> Swift interface
• Session-specific network connections

The above list provides a pretty good roadmap for what I'll be working on this week :)

[ronaldmannak]

@natclark can you take a look at the commit since all changes are in the JS part?

[jamierumbelow]

Holding in draft state until #9 is merged, will update this branch then.

[jamierumbelow]

Superseded by #13.