refs https://github.com/TryGhost/Ghost-Admin/pull/690, closes#1501, closes#2093, closes#4592, closes#4627, closes#4659, closes#5039, closes#5237, closes#5587, closes#5625, closes#5632, closes#5822, closes#5939, closes#6840, closes#7183, closes#7536
- replace custom showdown fork with markdown-it
- swaps showdown for markdown-it when rendering markdown
- match existing header ID behaviour
- allow headers without a space after the #s
- add duplicate header ID handling
- remove legacy markdown spec
- move markdown-it setup into markdown-converter util
- update mobiledoc specs to match markdown-it newline behaviour
- update data-generator HTML to match markdown-it newline behaviour
- fix Post "converts html to plaintext" test
- update rss spec to match markdown-it newline behaviour
- close almost all related showdown bugs
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