Allow an horizontal scrollbar

Related to #967 (closed) and #970 (closed)

This provides a simple, though inconvenient, workaround to any issues where something causes an horizontal overflow of the whole display: just display an horizontal scrollbar.

Bulma disables the horizontal scrollbar entirely by default, and always enables the vertical scrollbar, but we can set the horizontal scrollbar it to only show when there is some horizontal scrolling to do.

This at least allows the users to get unstuck when something overflows the viewport without having to know how to navigate using the Tab key.