Tinymce update with upstream

This PR updates our fork of TinyMCE to the latest upstream master. I made a clean fork of upstream and cherry-picked our commits over. I left out commits that fixed bugs already fixed in upstream. Commits cherry-picked over:

- Fixes HTML mangling while stripping data-mce helpers (https://github.com/Shopify/tinymce/commit/f423a2ac2dc045f68a7a10330d158689bf988a4e)
- Updated readme (https://github.com/Shopify/tinymce/commit/87865ae7cb4e4d225feaa731e82b03487e48dbdf)
- Added noparsing plugin (https://github.com/Shopify/tinymce/commit/41eff283fde5fc2cb1cfc80246f73bb63c29de13)
- Fixed compile error in noparsing plugin (https://github.com/Shopify/tinymce/commit/cb4abc50ee902b3e6833c571fac21edb39ac2cac)
- prevent serialization in setContent (https://github.com/Shopify/tinymce/commit/669d84153906bff1df0a0c69daacd5046220af52)
- prevent removal of empty elements in DOM parser (https://github.com/Shopify/tinymce/commit/e40de1345936720d29a122636ed3a1ba49268c53)
- Prevent bogus br from being added to empty RTE content (https://github.com/Shopify/tinymce/commit/e54091169f9f7342bbb1ae87f92c22f834371535)
- Set node to current context if undefined (https://github.com/Shopify/tinymce/commit/9234e6f360da97626ecadd3a323c9990235dcf2a)
- Add optional CSP header to Editor iframe (https://github.com/Shopify/tinymce/commit/f51b9c2040a980e54bfad6da62760fb330afe944)

There are some tests that test for things we [want/don't want in Admin]( https://github.com/Shopify/shopify/pull/8208). I commented them out, but would be happy to just delete them too:
- ['Remove redundant br elements'](https://github.com/Shopify/tinymce/blob/999ae52f6a0b28022d14438f346fb0bc48416c89/tests/tinymce/html/DomParser.js#L326)
- ['Invalid inline element with space before'](https://github.com/Shopify/tinymce/blob/999ae52f6a0b28022d14438f346fb0bc48416c89/tests/tinymce/html/DomParser.js#L483)
- ['Parse no attributes span before strong'](https://github.com/Shopify/tinymce/blob/999ae52f6a0b28022d14438f346fb0bc48416c89/tests/tinymce/html/SaxParser.js#L569)

Since this branch rewrites the history of master It'll have to be a force push.

@angelini @nsimmons