mediawiki.ui: Add right padding to .agora-field-styling()

It should have padding-right equal to padding-left (0.5em, previously had 0).

Followup to I7fdd9970.

Bug: 56805
Change-Id: I6652d68f8ed1be106731d1fa6b9a990dc6efe41f
Merge "mediawiki.ui: Add right padding to .agora-field-styling()"