Bug 30136: Add back to top button to OPAC

Bug 30136: Add back to top button to OPAC

Like in many websites, when scrolling down the page make a button appear to go back to top.
This can be very useful specially in search results page.

This patch adds to OPAC using Javascript a button with id "backtotop".
Button appears when 300 pixels from top are hidden.
This value may be adapted.

Test plan
1) Go to OPAC
2) Perform a search with many results
3) Scroll down
=> Back to top button appears
4) Manually scroll back to top
=> Button disappears
5) Scroll down and click on Back to top button
=> You get to top

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>