Mobile: glossary definitions off-centre and button sometimes escapes popover

[bryceerobertson]

It seems the "View full definition" button falls out if the preview text is sufficiently long.

[bryceerobertson]

@zarSou9 Looks great except one tiny thing worth fixing only if it's easy: for some glossary definitions (actually I only experienced it with "superintelligence") the "View full definition" text isn't centered in the button. I think it might be because the defined word is so long, making the button wider than normal or something.