Fixes #3665: Improved Gii CRUD generated code to support setting initial conditions via initializing search model

Fixes #6427: In case of invalid route web application now throws exception with "Page not found" instead of "Invalid Route"

docs/guide/input-file-upload.md - revised a bit [ci skip]

/docs/guide-ja/input-file-upload.md added [ci skip]

Fixes #6434: Added `yii\behaviors\SluggableBehavior::immutable` to support keeping the generated slug unchanged

Spanish guide Query Builder [skip ci]

/docs/guide-ja/input-validations.md added [ci skip]

docs/guide/input-validation.md - typo [ci skip]

[skip ci] Update start-looking-ahead.md

[Uk] Guide corrections [skip ci]

Adding the tag titled "yii2" on Stackoverflow to Community list.

fix  mistake small  [skip ci]