UI/API changes

NEW: Landing page for logged out users: intro, live network stats and tools slideshow
NEW: Measurements list: LatencyMON shortcut in actions column
NEW: Khipu: warn when prefixes are announced by more than one ASN
NEW: Anchor change request: check ASN/IP match, show ASN holder
CHANGE: Msm overview: much faster for large measurements, don't recalc when switching back from other tabs
CHANGE: Msm overview: stream results so large sets show sooner
CHANGE: Khipu: show count of results received while loading
CHANGE: Khipu: much faster loading and filtering for large datasets, faster v6 renders, better v6 IXP matching
CHANGE: Khipu: don't draw labels until zoomed in enough to read them
CHANGE: Khipu: rework country view for better performance
CHANGE: Khipu: exclude filter hides paths matching any term (instead of all)
CHANGE: LatencyMON: batch ping requests (not traceroutes), speed optimisations
CHANGE: LatencyMON: default country sort by distance
CHANGE: LatencyMON: chart title tooltips
CHANGE: Probe detail results: speed optimisations, only request uptime once
CHANGE: Maps: better centering when probes span the entire globe
FIX: Msm overview: show on map links
FIX: Community events card: fix cutoff when ongoing event
FIX: Khipu: popups partially drawing off-screen
TWEAK: Rename News Flash to RIPE Atlas News
TWEAK: Map watermark update and placement
TWEAK: Sponsors card fills its height