migrations module: remove config['migrations_path']

Remove the migrations_path config option.  We also notice that the removed
comment claims the path needs to be writeable - this is not true now, so
we can remove the corresponding check from the installer.

Even before the new directory structure, it didn't make any sense to change
the migration path.  (Bonfire looked in a "core" subdirectory for it's own
migrations, so you would have had to copy that over as well).

In the new directory structure, the application has its own migrations
directory.  But the config didn't allow you to change that.  It only
allowed you to change the bonfire/migrations directory!  (The installer
uses the corresponding constructor parameter, but that doesn't mean we
need a config option, and we could easily get rid of that if we wanted to
anyway).
migrations module: remove unused config['migrations_enabled']

Dead code.  (The config that works is migrate.auto_core.  The stock
application.php has config['migrate.auto_core'] = TRUE).
migrations module: remove unused config['migrations_version']

This only applies to bonfire core migrations, so it should never have been
used by applications.

This was the last option in the file migrations/config/migrations.php, so
I remove the file.
migrations library: remove unconditional debug line
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 :).
migrations library: fix broken install() method

install() wasn't updated to match the new directory layout.  Also it's
redundant with the get_latest_version() method.  The extra filtering in
install() is interesting, but being inconsistent about that is obviously a
Bad Idea.

Fix get_latest_version() to use the stricter filtering, (but still use
the correct paths).  Then fix install() so it just calls
get_latest_version().
Migrations library: tidyup

 - Move related methods a bit closer together
 - Trivial refactor of auto_latest() to use install()
 - Nitpick comment consistency.
migrations library: remove set_path() method

The migrations controller used set_path() for module migrations, but that
wasn't needed, at least not any more.

The problem with setting the path is there are actually three different
paths.  (This is more obvious following the new directory structure, but
I think the same problem applied before as well).
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)