Add feature flag to default Rapid Diffs on MR Changes tab

What

Introduces the rapid_diffs_default_on_mr_show feature flag — the first step of enabling Rapid Diffs by default on the merge request Changes tab (#602723 (closed)).

• Flag off (default): the Changes tab keeps today's opt-in Rapid Diffs behaviour. No change for users.
• Flag on: Rapid Diffs becomes the default; users opt out via the rapid_diffs_enabled cookie.

/diffs routes to a single action and the controller resolves which page to render, so the flag is evaluated once with current_user and an actor-based rollout behaves as expected. The Rapid Diffs toggle reflects the actual page mode. The flag is stubbed off suite-wide in spec_helper, so the existing legacy-diffs specs are unchanged; the spec migration to Rapid Diffs lands in a follow-up MR.

Feature flag

rapid_diffs_default_on_mr_show (gitlab_com_derisk) — off by default.

Rollout issue: #609355.

Related to #602723 (closed).