port to WebExtensions

[nobodxbodon]

According to https://compatibility-lookup.services.mozilla.com/:

WebExtensions are becoming the standard for add-on development in Firefox. It will be the only type of extension supported in Firefox by the end of 2017.

Pitifully sidebar isn't supported by WebExtensions:
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Comparison_with_the_Add-on_SDK#UI_elements

Plan is to change UI to popup panel or separate tab like FromWhereToWhere for Chrome

[nobodxbodon]

Seems there'll be sidebar API for WebExtensions: https://wiki.mozilla.org/WebExtensions/sidebarAction

So the main focus of this issue in short term is the other deprecating APIs if any.

[nobodxbodon]

According to sdk doc, 57.0 will stop loading non-webExtension extensions.

According to release schedule, merge date of 57.0 beta is 2017-09-20, and release date is 2017-11-14.

Setting milestone to 2017-09-20.

Just successfully tested chrome extension with this fix in FF 56.0b2. Still need to add sidebar and other features like icon of web pages.

Current sidebar API: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/user_interface/Sidebars

[nobodxbodon]

Done in new version at https://github.com/program-in-chinese/HistoryInThreads_WebExtension