Use variables for translatable strings in hidden legends, to match visible text

[sabernhardt]

Using a variable, wherever possible, the visually hidden legend text matches the visible text that describes the same fieldset.

Strings that do not use a variable but still remove the "Hidden accessibility text" comment:

• 'Target' in the Links metabox, because it matches the metabox heading in edit-link-form.php
• 'Administration Color Scheme' in the profile screens, because the table header is in user-edit.php

Trac 63620

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