Add some tests for grammars.yml

It must be sorted, because it will get re-sorted when
script/download-grammars is next run and that would clutter up diffs.
And it must not contain any duplicate scopes.
Use raw links as little as possible

It's much cleaner and more robust to specify the repo URL instead of a
URL to an individual file. If the file gets moved we'll still be able to
find it if we're using the repo URL.

Where we do need raw links, we now use github.com/owner/repo/raw/*
because it looks a bit nicer and sorts better with non-raw links.
Sort grammars.yml
Remove the WebIDL grammar

This grammar uses the same scope name (source.idl) as our existing IDL
grammar. The wrong scope name was listed in grammars.yml which masked
this problem. https://github.com/andik/IDL-Syntax/pull/2 tracks getting
the grammar to use a different scope name; in the meantime we just won't
highlight these files.
Update grammars.yml

I just ran script/download-grammars and these are the changes it made.
The most notable one is that we had the wrong scope listed for Markdown.
Merge pull request #1891 from github/fix-grammars

Fix some bugs in grammars.yml