Screen reader class - audit of existing selectors and their labels

[hnla]

Ticket covers the task of auditing all existing BP screens where we might be using and hiding form labels to add new bp-screen-reader-text class, and removing display: none rulesets where they may exist. (as suggested in #5513)

[hnla]

Styles audited and all instances of label/inputs checked visually for any previous hidden label text, none exist other than out recent additions and screen reader class for messages/notifications.

messages search input though is wrapped in a label without any text, this should be updated to include text and have class added for screen reader text to hide visually and preserve existing installations expectation with element.

[hnla]

Update messages search for label text and screen reader class.

[boonebgorges]

In 9240:

Improve <label> markup for mesage search form.

• Associate the label properly with the labeled element.
• Add bp-screen-reader-text class for better consistency with other similar text in BP templates.

Props hnla.
Fixes #6010.