fix(dropdown): remove `C` restrictions to avoid conflicts

Fixes #2156

BREAKING CHANGES: Both `dropdown` and `dropdown-toggle` are only attribute directives

Before:
  ```html
  <btutton class="dropdown-toggle" ...>
  ```
  After:
  ```html
  <btutton class="dropdown-toggle" dropdown-toggle ...>
  ```

Closes #2170
docs(CONTRIBUTING.md): clarify GitHub issues handling policy