Prevent unstyled flashes of Kickoff content and team badge images

[kilgor-trout-c22]

What

Fixes two brief unstyled flashes introduced by moving the front-end styles into an asset bundle (#34) and the flags to SVG images (#37), both most visible in Firefox:

• Badge images flashing at full content width — the Twemoji SVGs carry no intrinsic size, so before the stylesheet applies, the first flag image briefly rendered at full width. The badge <img> tags (flag and logo) now carry explicit width/height attributes; CSS still controls the final size.
• Unstyled Kickoff content on in-app navigation — HumHub navigates via Pjax, which swaps the page content in before a stylesheet registered by the target view has loaded. The Kickoff stylesheet (~8 KB, long-cached) is now registered on every full page load via a LayoutAddons hook — the same pattern core modules use — so the styles are always present before the swap. Per-view registrations stay as fallback for direct loads.