Previously these parts have been spread out in several, heavily
coupled hooks and static configurations, making it very hard to get an
overview of how it all actually works. Now we instead do everything in
one place, and generate things programatically as much as possible,
which makes supporting another locale much simpler (just adding a
single line with a few pieces of needed info!). Consequently this
commit also adds better localization for every locale supported by the
Tor Browser (added: ar, es, fa, ko, nl, pl, ru, tr, vi and zh_CN)
instead of the euro-centric (w.r.t. actually adding localization) ones
we had picked before. Also, we'll get build level errors for many
types of errors, so they're caugt early.

This commit may seem like a gigantic, non-atomic beast, but splitting
it would be hard, and it actually mostly removes stuff. Its design is
simple: We have a description file that, for each supported Tor
Browser locale, has the extra pieces of info needed, like the
localized name of the language, the parameters needed for certain
search engines etc. Then we apply them to:

* a templates for the amnesia branding locale file, and put in the
  relevant values there so the default search engine is selected, the
  correct spellchecker (if installed) is pre-selected, and our
  homepage is localized (if supported).

* a template for each of the search engines we want to localize
  (currently Start Page and Disconnect.me -- we could do Google but
  most (all?) locales get a localized one from the Iceweasel
  localization packages) and generate localized ones.

Also, following the Tor Browser's recent switch, we now use
Disconnect.me as the default search engine, although we localize it
for each supported locale.

Will-fix: #9309