Clean up border-radius

[Xyene]

We have a lot of different border-radius values in use throughout the site, sometimes in conflicting places.

• Paginators use 4px, tables use 6px. This looks weird when both are side-by-side; I think tables should be 4px too.

• Comment boxes use 4px, but profile images are rounded to 6px.

• Buttons use 4px

• Code blocks use 4px

• The submission table has no radius, despite being surrounded by elements that do

Consider, instead: