improve browser resource loading

[cezaraugusto]

This is somehow related to #3025 but split as it's for resource loading only

• make use of resource hints (ref: https://www.w3.org/TR/resource-hints/)
• Pre-load common assets (preload/prefetch)
• Cache resources? (needs investigation)
• Audit flow/reflow/repaint (tracked alone in Audit DOM APIs that do layout reflows #10027)
• audit use cases for async/defer
• better code split for webpack (tracked in Tracking: Improve webpack build time/size #11476)
• inline SVG
• Compress images (ref Tracking: Improve webpack build time/size #11476 (comment))
• remove unneeded fonts (mostly FA)
• async module loading? (needs investigation)
• decide when to use inline data: URIs and when to load via separate file (completed with Tighten webpack rules for which assets to inline as data URIs #12225)