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Second try on this commit.

Follow up to #12958.

Fixture files are passed through an ERB renderer before being read as
YAML. The rendering is currently done in the context of the main object,
so method definitons leak into other fixtures, and there is no clean
place to define fixture helpers.

After this commit, the ERB renderer will use a new subclass of
ActiveRecord::FixtureSet.context_class each time a fixture is rendered.

Also wrap surrounding text to 80 chars.

[ci skip] Closes #13132

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Conflicts:
	guides/source/4_1_release_notes.md

he or she => they

related  #49ff20d9

Fixes: #13108
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removed unnecessary test case and improved test case for belongs_to having invalid  options

The section that mentions after_commit and after_rollback will swallow exceptions should me made clear. Made that section to be a  instead of a regular paragraph.

/cc @fxn

Extract **notable changes**, **deprecations** and **removals** from
each CHANGELOG.

I tried to reference the commits and pull requests for new features
and deprecations.

In the process I also made some minor changes to the CHANGELOGS.

The 4_1_release_notes guide is declared WIP.