Make sure Place Block buttons identify as buttons in Block layout UI.

Problem/Motivation

The "place block" buttons on the Block Layout UI (admin/structure/block) are made with HTML anchor tags.

These are announced as a "link" by most screen readers, suggesting that the user will be directed to a new page. The actual behaviour is better described as a "button", to suggest that something will happen on the current page.

Proposed resolution

Either:

1. Preferably, use a <button> element for the the "Place block" buttons on the Block Layout UI.
2. If this is difficult, adding role="button" to the existing anchor tag will suffice, but we would also need to ensure it is activated by enter OR the spacebar key, as a <button> would be. (Anchor elements are normally only activated by the enter key.)

Remaining tasks

Write a patch.

User interface changes

• No visual changes intended.
• Improve the semantics conveyed to assistive technology.

.

API changes

None expected.

Data model changes

None expected.