LP#1268636 TPAC initial navigation headers

LP#1268636 TPAC initial navigation headers

Added a screen-reader only CSS class ("sr-only") for adding content
which should only be read by screen readers and is otherwise invisible.

Added hidden generic <h1> and page-specific <h2> entries to improve
web accessibility navigation.

Additional content-specific headers (h3 and down) are recommended for
content grouping, but not included in this commit.

Signed-off-by: Bill Erickson <berick@esilibrary.com>
Signed-off-by: Dan Scott <dscott@laurentian.ca>