Add buttons to show/hide SVG code

I will then add similar buttons for CSV exports, after extracting
a utility function to share code for behavior common to the 3 modules.
Use the block id as prefix in ids of related buttons and presenter

This is an intermediate step in the refactoring of the code before
extracting common functionality for reuse.
Extract function showHide() to new module interactivity-utils.js
Add buttons to show/hide CSV exports for Authors and Categories
Add 2em margin around the page for breathing

I had to switch the apply an inline-block display instead of a float
left style to the columns on the left (column1of2). Otherwise, the
margin was not applied to float elements, which overlapped and hid
the margin instead.
Merge branch 'master' into gh-pages