feat: load Web UI from IPFS

[lidel]

This is a prerequisite for #679, so I am merging in fast-track mode to create a new release and fix #679 ASAP. cc ipfs/ipfs-webui#959
Feel free to comment – will address any concerns in separate PRs.

Before, webui was bundled with extension itself. Unfortunately this
caused issues with reviews at addon (webui build was not reproducible).

This PR replaces bundled webui with one loaded from custom gateway.

• To make it work we modify HTTP headers to make cross-origin requests
  from 'blessed' webuiRootUrl work without changing default go-ipfs configuration.
• To improve UX, content script sets ipfsApi in webui's localStorage
  to the same endpoint as one defined in Companion (unless it was already
  customized by the user).
  • Switched programmatic content script injections from browser.tabs.onUpdated to browser.webNavigation.onDOMContentLoaded it executes only once and in case of linkify experiment DOM updates are handled via MutationObserver anyway
• Sadly Web UI for js-ipfs had to be disabled for now (it may come back thanks to lunet experiment from The Future of "accessing API of remote IPFS node" in-web-browsers#137)

Digressions / notes to self:

• The amount of differences in HTTP header and CORS handling between Firefox and Chrome in WebExtension context is baffling :'(
  • Example: Chrome requires additional handling of values in preflight request, namely value from Access-Control-Request-Headers needs to be manually restored in Access-Control-Allow-Headers (Firefox allowed whitelisting cross-origin webui access without this step)
  • Fun fact: Chrome 72 removed access to arbitrary headers from webRequest. If one wants to peek at Referer header, they need to add extraHeaders to listener's constructor. This works only in Chrome – Firefox breaks, so now one needs to provide a different arguments to browser.webRequest.onBeforeSendHeaders.addListener depending on which browser is the runtime