Closes #9483.

There are SQL Queries that can't run inside a transaction. Since
the Migrator used to wrap all Migrations inside a transaction there
was no way to run these queries within a migration.

This patch adds `self.disable_ddl_transaction!` to the migration to
turn transactions off when necessary.

Sometimes you want to create a table without an associated model and
test, which is also not a join table. With this commit, you can now
do that.

Example:

    rails g migration create_posts title:string
or
    rails g migration CreatePosts title:string

This commit also moves the template the model generator uses for the
migration to the migration templates folder, as it seems a more
sensible place for it now that it is shared code.

Explain that it doesn't create indices by default and
that it also has a block form.

We should mention that the ordering matters with regards
to timestamps, and not imply that it's just for uniqueness
purposes.

Closes #8610.

This introduces a bunch of editing/re-writing to the migrations guide.
There were a bunch of small changes, and a few larger ones:

* mysql does support transactions.
* Add a section about db/seeds.rb
* Largely re-wrote the first few sections

We have three or four different introduction sentences to the guides.
After this commit, we use the same one everywhere.

Other guides that talk about Active Record specific features include the
name of the library in the title, this one should too for consistency.

Talked with @fxn about this. Bullet points should have periods at the ends.

The wording of these was a bit off, so I fixed them.

Most of the guides have a few simple sentences describing what they will show
you at the top. This one had a few big paragraphs. I've moved those paragraphs
down to an introductory one, and written a new smaller one for the introduction.
This makes this guide more consistent with the others.

Changed comment section to be more explicit. I had an issue with rolling back a migration and couldn't understand why 'rake db:reset' didn't work.

This reverts commit f5b9ed4f.

REASON: Translation work can not be done in the docrails repository

This is an attempt to address issue #6939, where an earlier migration
added a column to the database, and a later migration uses a model and
references that column.

When both migrations were run together with `rake db:migrate` the column
information in memory still referenced the old table structure.

Running the migrations separately fixed this, as a new connection was
then established before referencing the model. Explicitly calling
`reset_column_information` is a more reliable workaround.

The feature is available after Rails 3.1 and this is no longer a "new" feature.