This updates the datagrids code to introduce a #rb-ns-ui.datagrids

namespace containing the various margins, paddings, colors, and other

styles used by our datagrid. This builds upon the new

#djblets-ns-ui.datagrids namespace in Djblets, extending those to give

us a single place where we can look up all variables.
Some of the datagrid code has been shuffled around in the process. The

datagrid classes have been made flatter and more reusable. Some styles

had to be updated to account for this, as specificity changed.
It's not a full clean-up, and documentation is not being added at this

stage. Eventually we'll want to rethink the component structure and

styling of datagrids in Djblets, and rethink some of the add-on classes

we've introduced here, but that's a change for another day.

Checked all the datagrids and the affected columns, making sure it all

appeared to render correctly.