Migrate simple-prefs to WebExtension storage

[lidel]

It is now possible to embed small WebExtension (WX) within legacy SDK add-on.
We should add this in master as soon as storage keys in webextensions branch are mature enough. Then make official, transitional release (eg. v1.6.0).

This will gracefully migrate user preferences (eg. Custom Gateway/API URLs) to the WX namespace, and when WX time comes, everything will work as expected.

PoC: https://github.com/mdn/webextensions-examples/tree/master/embedded-webextension-sdk
Ref. https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Embedded_WebExtensions

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Migration Tasks

• create analogous options in 2.x.x (WebExtension) for all settings in 1.x.x (legacy SDK)
• add settings migration in legacy-sdk branch (embed WebExtension stub that reads settings and saves them under WebExtension storage)
• release hybrid addon from legacy-sdk as 1.6.0

[lidel]

Prerequisite PR: #157 introduces JPM 1.1.2 which delivers support for hybrid SDK addons.

• https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Embedded_WebExtensions

This means you can migrate a legacy add-on to WebExtensions one piece at a time, and have a fully functioning add-on at every step. In particular, it enables you to migrate stored data from a legacy add-on to a WebExtension, by writing an intermediate hybrid add-on that reads the data using the legacy APIs (for example, simple-prefs or the preferences service) and writes it using the WebExtension APIs (for example, storage).