34 new icons, plus some renames

[mdo]

Isolates the SVGs from #141, while keeping the content changes intact over there.

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• New: octagon, octagon-half, octagon-fill (fixes [icon request] empty octagon shape #104)
• New: clipboard, clipboard-data
• New: layers, layers-half, layers-fill
• New: briefcase
• New: double chevrons (chevron-double-down, chevron-double-left, chevron-double-right, chevron-double-up) (fixes [icon request] double chevron icons #109)
• New: funnel, funnel-fill
• New: grid-1x2, grid-1x2-fill
• New: grid-3x2-gap, grid-3x2-gap-fill
• New: grid-3x2
• New: grid-3x3
• New: graphics glyphs (exclude, intersect, subtract, union, circle-square)
• New: layout-sidebar-nested
• New: layout-three-columns
• New: layout-wtf
• New: newspaper
• New: paperclip
• New: people-circle
• New: diamond, diamond-fill
• Renamed: columns-gutters to columns-gap
• Renamed: diamond to gem, because diamond shapes
• Fixed: document-* classes changed to file-* to match the rename from Icon permalinks #137