Convert Quick Upload screen to Sidebar action

[lidel]

Right now Quick Upload creates temporary tab.
Sidebar would be better from UX perspective, as user would not have to switch contexts.

Firefox 54 provides Sidebar API: https://blog.mozilla.org/addons/2017/03/13/webextensions-firefox-54/

[lidel]

Turns out Sidebar API is supported only by Firefox and Opera 👎

I think it is better to stay at current tab-based approach:
it works and looks the same in both Firefox and Chrome.
Making different user experience under Firefox would introduce unnecessary maintenance cost and a new surface for bugs.

Reference: https://developer.mozilla.org/en-US/Add-ons/WebExtensions/manifest.json/sidebar_action#Browser_compatibility