Add foreign_type option for polymorphic has_one and has_many.

To be possible to use a custom column name to save/read the polymorphic
associated type in a has_many or has_one polymorphic association, now users
can use the option :foreign_type to inform in what column the associated object
type will be saved.
Remove unused "Developer" fixtures from tests

The `RecursiveCallbackDeveloper` and `ImmutableMethodDeveloper` classes
are not used anymore in tests, and neither is the `@cancelled` variable.
Merge pull request #17972 from claudiob/remove-unused-callback-fixtures

Remove unused "Developer" fixtures from tests
Unused csv fixture file
Fix options overwritten by super
Merge pull request #17975 from merongivian/add_test_for_search_field

Fix options overwritten by super
Adding a tip in the guides about possible database permission problems on
loading fixtures to the database.

Reference #17542
Merge pull request #17982 from andreynering/testing-documentation

Testing Guides: adding TIP about database permission problems [ci skip]
Add guides for `required` model generator option.

Related to this PR #16062

[ci skip]
Merge pull request #17988 from caike/master

Add guides for `required` model generator option
Adding information about 'allow_nil' option for 'numericality' [ci skip]
Merge pull request #17987 from andreynering/ar-validation-doc

Adding information about 'allow_nil' option for 'numericality' [ci skip]
[ci skip] fix typo in MessageVerifier#verify docs
Merge pull request #17989 from y-yagi/patch-1

[ci skip] fix typo in MessageVerifier#verify docs
Merge pull request #17970 from ulissesalmeida/foreign-type-has-many-has-one

Add foreign_type option for polymorphic has_one and has_many.