Support creating a table migration generator

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.

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.

Removed the bit about `SCRIPT_NAME` handling, since setting
`default_url_options[:script_name]` does work correctly.
We ran into an issue with it but it was an app bug.

Add some more documentation to the upgrade guide

[ci skip]

Wrong exception is occured when raising no translatable exception

Closes #8617

[ci skip]

Fix for #9360 content_for regression

Support PostgreSQL specific column types when using `change_table`.

We use `TableDefinition` for `#create_table` and `Table` for `#change_table`.
The PostgreSQL sepcific types were only defined on `TableDefinition` so they needed to be added to `Table` as well.

Closes #9480.

We use `TableDefinition` for `#create_table` and `Table` for `#change_table`.
The PostgreSQL sepcifc types were only defined on `TableDefinition` so I
also added them to `Table`.

`format: true` does not override existing format constraints.

Keep in sync with 3-2-stable.

Closes #9466.

Passing `format: true` used to override the constraints: { format: /json/ }
with `/.+/`. This patch only sets the format if there is no constraint present.

This reverts commit 408227d9, reversing
changes made to dca0b57d.

Closes #9386

Fix calculation of db_runtime property in
ActiveRecord::Railties::ControllerRuntime#cleanup_view_runtime.

Previously, after raising ActionView::MissingTemplate, db_runtime was
not populated.

Closes #9218, Fixes #9215.

Conflicts:
	activerecord/CHANGELOG.md

Remove duplicated number_helper tests in AP. They are already in AS.

With 155cd5e6 the number_helpers were moved into AS all the tests were copied over but the tests in AP were not deleted. This is confusing.

This removes all duplicated tests and reorganized the tests in AP to only test the functionality, that is added in AP.

Also cleanup some of the number helper tests.

* use 1.9 style hash syntax
* don't use brances on assert_equal
* prefere " over '

With 155cd5e6 the number_helpers were moved into AS all the tests were copied over
but the tests in AP were not deleted. This is confusing.

I removed all duplicated tests and reorganized the tests in AP to only test the
functionality, that is added in AP.