Adjust margin between h2 and special page table element

With the recent typography update, this margin is much too small.

.mw-specialpages-table applies "margin-top: -1em;" style which
doesn't fit with the vector styles. So this provides an override
for vector.

Bug: 63312
Change-Id: I18a711802a18b5976cf749d76138a1a6f59e0a4e
Merge "Adjust margin between h2 and special page table element"