Add structure-extensions.md to translate into Russian
updated doc [skip ci]
fixed console guide and docs of FixtureController
Minor text adjustments
Docs about testing
Rewritten testing introduction, added environment setup to README.md, removed empty pages that were not referenced
expanded db-migration doc
Update db-migrations.md

fix a typo.
Fixes #4800.
fix bugs of BlameableBehavior in console application. In console application, \yii\console\Application doesn't have `getUser()` method. Use BlameableBehavior in console application will cause an exception.
Fixes #4736
First pass of edits
structure-extensions.md is translated to Russian
Added more notes and a book reference to testing intro
Replaced Amazon link with ISBN
Adjusted testing namespaces in application templates
Added description to each test config of advanced application
Added faker to basic app console commands, adjusted readme to mention database setup
fixes for translation of file 'structure-extensions.md' to Russian
fixes for translation of file 'structure-extensions.md' to Russian