MediaWiki UI: Use em's for the button width

In mobile iOS this causes overlap of buttons with footer text
on our Wikigrok widget.

We use
em's to create a margin for the button but since the button is defined
in pixels when zooming out of the page this causes overlap.

Bug: 71577
Change-Id: I0332b925a3f09c7f924aa48bd636f7fb19743ff7
Merge "MediaWiki UI: Use em's for the button width"