css: fix vertical alignment in repo and PR summaries etc.

In several table-like layouts (not actual HTML tables) with separate
"label" and "value" columns, this 10px margin-top (which only applies
to the "value") caused the "cells" to be out of alignment:

- The repository summary (clone URL, description, etc.)
- The pull request "summary" (description, origin, target, etc.)
- Settings pages of all sorts