Adding a new additional validation rule in the gii-crud generator enforcing the expected format.

Fix bootstrap templates

setting writable folders in init-script

[ci skip] overwrite -> override

fixes #3252

fixes #3252

also improved unit tests for querybuilder buildLikeCondition

corrected app dir example and removed license code from example.

[Guide] Fix link

fix typo in the word Semantic

Improved the advanced app template documentation.

Fixes #3268: Fixed the bug that the schema name in a table name was not respected by `yii\db\mysql\Schema`

Not sure about the “for example…” paragraph under “usage”. Doesn’t read
well currently.

Fixes #2906: Added support for using conditional comments for js and css files registered through asset bundles and Html helper

[ci skip]

fixes #3255

Q: When attaching event handlers in the config, does it have to be an
anonymous function?

Moved “triggering” after “attaching”.

Cut a sentence that seemed both redundant and vague. Could just be me.