squash! MB 213: Customize the search engines list

MB 328: Refactor the search engine patch.

Upstream switched to a completely different search engine configuration
between ESR 115 and ESR 128.
We moved our configuration to a couple of JSON files that do not follow
upstream's schemas, as they are overcomplicated for our needs.

Also, we keep the old search engine extensions for now, as upstream
also kept them, and planned of removing them with Bug 1885953.