Exposed forms in a block missing custom CSS class

Problem/Motivation

If you create a View with an exposed form in a block, add a CSS class, and place the block, the class is not included in the final output.

Steps to reproduce

1. Create a View with a filter
2. Enable Exposed form in a block
3. Add CSS class: my-class
4. Place the block on a page
5. Check the page source, see the block is present, and note the missing my-class class

Proposed resolution

Fix the code in Views, so that any custom classes are output.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet