plugins/slate: implemented HTML pasting (react-page#483)

Signed-off-by: Oliver Habryka <[email protected]>

all: resolve remaining dnd context issues (react-page#467)

ui: fix dragdrop context provider

plugins/slate: pass parse5.parseFragment to html deserialization

plugins/slate: pass parse5.parseFragment to html deserialization

plugins/slate: add parse5 to hooks

all: resolve ci issues

plugins/slate: annotate br serialization arguments

core/grid: resolve firefox flexbox issue

core: add editables reducer test