Improve consistency of block names

[nylen]

There are a few blocks that have sub-optimal names due to our server-side parsing not handling dashes properly:

• core/latestposts should be core/latest-posts
• core/embedcollegehumor (and other embed blocks) should be core/embed-collegehumor or core-embed/collegehumor. Per Slack discussion with @mtias, and because there are enough of these blocks that it makes sense for them to live in their own "namespace", I've gone with the latter.

With the new PHP parser in #1152 we can resolve this ambiguity, with plenty of test cases, and set good standards for other block implementers.

[nylen]

#882 is the related issue I was looking for earlier.

It's probably easiest to review this PR without the fixture file changes and without #1152 in the mix, as follows:

git diff 2f8f07b...0074b3c blocks/library/ blocks/test/full-content.js

[notnownikki]

As far as the renaming goes this is 👍 from me, this makes much more sense than the smooshed together names.

I'm not familiar yet with how pegjs works, so I wont mark this as approved, just because I don't yet know enough to properly review that part.

[nylen]

That's fine, that's a separate PR anyway (#1152).

[nylen]

Merging to get this breaking change in sooner rather than later.