Homepage|Style: Added support for high-DPI displays; improved forum font scaling

The homepage now rescales responsively for high-DPI and the margin
around the page is removed on smaller-display devices (e.g., mobile).
The homepage now scales down to 355 "CSS pixels" in width.
Merge branch 'master' of github.com:skyjake/Doomsday-Engine
Fixed|Renderer: BspLeaf::MissingClusterError drawing a particle model

Particle models are drawn from the interpolated origin, which, may
lie in a degenerate BSP leaf even though the actual origin is within
the map. Consequently the BspLeaf given to R_CollectAffectingLights()
may be degenerate.
Merge branch 'master' into gl2-lensflare