Deprecate drupal_rewrite_settings()

Problem/Motivation

Convert the procedural function of setting the editor into a class.

Proposed resolution

Make a class with a single entry point for writing settings. Other internal functions convert into protected methods.

Remaining tasks

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User interface changes

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API changes

See CR. Function drupal_rewrite_settings() was replaced by SettingsEditor::rewrite() method.

Data model changes

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Release notes snippet

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