migrations library: add missing language lines (from installer)

I'm dumping these in application_lang.  It just saves a bit of thinking,
w.r.t sorting out the copy of the library used by the installer.
migrations library: installer doesn't have application_helper

(A copy of) the migrations library is used by the installer.  The installer
doesn't have application_helper (selected helpers are placed in
installer_helper).

Fortunately the applications_helper is in config/autoload.php, so
there's actually no need for the migrations library to explicitly depend on
application_helper.

Again, the aim is to make the installer slightly easier to maintain :).
install: update Migrations library

From newest to oldest, I see
 - Language lines moved to application_lang.php
 - Recent removal of unused config options
 - Fix name of 'migrations_path' param
   (and update the callsite in Installer_lib.php).
 - Use the "new" layout of the schema_version table
   (see 012_Migration_schema_change.php)