[meta] Merge repo into web-platform-tests

[gsnedders]

From https://lists.w3.org/Archives/Public/www-style/2016Oct/0088.html:

• Land Add support for CSS WG-style manifests wpt-tools#90 into wpt-tools so that the web-platform-tests tools create an accurate manifest (i.e., list of tests) for the CSS testsuite.
• Add a number of lints to wpt-tools, for the (currently non-existent) css subdirectory, to ensure that the build system keeps working (primarily we need lints to ensure that we have no duplicate file-extensionless-basenames that aren't byte-for-byte identical and that all files referenced by test files are in an adjacent support or reference directory, with a couple of exceptions).
• Ensure web-platform-tests's documentation is up-to-date and cohesive, both for submitting tests and reviewing them. Especially make sure it's easier to find documentation than it is currently!
• Make https://hg.csswg.org/test/ and http://test.csswg.org/shepherd/ read-only. (This only depends on making sure we have something in way of GitHub documentation; [meta] Get GitHub as the only read/write source for the repo and issues #1101 )
• Merge csswg-test into web-platform-tests, in a css/ subdirectory, maintaining all csswg-test history, with w3ctestlib and apiclient being moved over to git submodules.
• Move over, at the very least, all open issues and PRs from the csswg-test repository.
• (Sometime in the more distant future) drop the current build system and the lints we had for its requirements.

[gsnedders]

The merge itself is now done. WOOOOO.

[wpt-issue-mover]

This issue has been moved to web-platform-tests/wpt#5486; please continue all discussion there.