Views exposed filter block doesn't have a title element

Problem/Motivation

The block used for exposed filters on content overview pages such as /all-topics does not contain a heading element.

Proposed resolution

I think using an h2 element for this would be a good idea since it's usually used on a page whose primary purpose is to filter the content. To make styling easier we can also move the title classes to be on the heading.

Replace <header{{ title_attributes.addClass('complementary-title') }}>{{ label }}</header> with <header><h2 {{ title_attributes.addClass('complementary-title') }}>{{ label }}</h2></header>

User interface changes

The header of a filter block is now properly marked as heading. No visual changes.

API changes

None

Data model changes

None