mediawiki.ui: Ensure that buttons and inputs are vertically aligned

Currently height of a mediawiki.ui button computes to 28px, but the
height of an input to 26px. Use vertical-align: middle; to make both
display at the center of line-height instead of at the top.

Followup to I7fdd9970.

Change-Id: Id4b45c57bae75bf1c5f56f5c43c714d91564dc15
Merge "mediawiki.ui: Ensure that buttons and inputs are vertically aligned"