Shared block: the Edit field is unlabelled

[afercia]

Any UI controls and form fields must have a properly associated label. Visible text labels are always preferable, alternatively a visually hidden <label> element or an aria-bale can be used.

Rationale:

• all users need to clearly understand what a form field is about
• screen reader users need to know what the form field is about when they land on it
• speech recognition software need the label to be visible so they're able to voice a proper command "Click {field name}" to move to the field

When changing a block to a Shared block, an UI appears to give the new Shared block a name:

This field misses a label.

Strongly recommended: tests to check the presence of labels, see #6098