Fix page links in mobile reading view

[dvglc]

In the mobile version of the reading view, page links tend to slide off the left border of the screen.

[dvglc]

I think this is a somewhat larger design issue, since the question ist how to represent page anchors somehow in the text (and not to the left of the text, as is currently the case). A possible solution might be special symbols (such as pipes "|") that would function as links on click, but I'm not sure if those page anchors would be visible enough.

In the very short term, we could simply hide links with (very) small screens. The Mirador view is almost unusable with very small screens anyway, so I think hiding it altogether wouldn't be a problem.