Activity calendar Pt 1: Add feature flag and placeholder entry point

Part 1 of 7 splitting !236549 (Vue profile activity calendar migration).

Introduces the :vue_profile_activity_calendar feature flag (default off) plus the minimal entry point:

• The FF-gated .vue-user-calendar container in _overview.html.haml providing calendar_path, activity_path, and utc_offset data attributes (with the permanent .activities-block min-height and a temporary gl-bg-strong placeholder styling in calendar.scss)
• A guard in user_tabs.js that skips the legacy D3 calendar and legacy activity feed when the flag is on, and instead mounts a minimal temporary Vue app that renders the provided data raw (unstyled) — proving the entry point and data flow end-to-end. It is replaced by the real calendar component in Pt 4

Merge order: pt-1 → pt-2 / pt-3 → pt-4 → pt-5 → pt-6 → pt-7.

Before	After
FF enabled

Test

• Enable FF https://gdk.test:3000/rails/features/vue_profile_activity_calendar
• https://gdk.test:3000/root

Reference (draft MR with the whole changes): !236549

Part of Modernize the profile activity calendar (gitlab-org/design-strategy/craft-quotient#3)