Omnibox: Add DDG, Startpage, Disconnect, Youtube, Twitter; remove Amazon, eBay, bing

eBay and Amazon don't treat Tor users very well. Accounts often get locked and
payments reversed.

Also:
Bug 16322: Update DuckDuckGo search engine

We are replacing the clearnet URL with an onion service one (thanks to a
patch by a cypherpunk) and are removing the duplicated DDG search
engine. Duplicating DDG happend due to bug 1061736 where Mozilla
included DDG itself into Firefox. Interestingly, this caused breaking
the DDG search if JavaScript is disabled as the Mozilla engine, which
gets loaded earlier, does not use the html version of the search page.
Moreover, the Mozilla engine tracked where the users were searching from
by adding a respective parameter to the search query. We got rid of that
feature as well.

Also:
This fixes bug 20809: the DuckDuckGo team has changed its server-side
code in a way that lets users with JavaScript enabled use the default
landing page while those without JavaScript available get redirected
directly to the non-JS page. We adapt the search engine URLs
accordingly.

Also fixes bug 29798 by making sure we only specify the Google search
engine we actually ship an .xml file for.

Also regression tests.

squash! Omnibox: Add DDG, Startpage, Disconnect, Youtube, Twitter; remove Amazon, eBay, bing

Bug 40494: Update Startpage search provider

squash! Omnibox: Add DDG, Startpage, Disconnect, Youtube, Twitter; remove Amazon, eBay, bing

Bug 40438: Add Blockchair as a search engine

Bug 33342: Avoid disconnect search addon error after removal.

We removed the addon in #32767, but it was still being loaded
from addonStartup.json.lz4 and throwing an error on startup
because its resource: location is not available anymore.