Move to fixed width column gaps for Overview in merge request

Proposal

Change the column-gap from 3% to 32px.

Rationale: Consistency of page layout

Source: #463598 (comment 2049497575)

HUGE thanks for fixing the problem that still exists in MRs, where when expanding the window, the gutter between the main content and the sidebar also expands. In work items, the sidebar and gutter are properly fixed 👍