Update COPYING

This updates COPYING to contain more information about licensing, by including information removed from README in this patch: https://gerrit.wikimedia.org/r/#/c/76643/1

I'm afraid the line-wrapping was done by hand; please feel free to LART me for that.

In general, I kept the text essentially the same, but there are a few somewhat substantive changes:
- changed URL from GPL (unspecified version, pointing currently to v3) to GPL v2, to match the text
- removed statement that the Foundation does not hold any rights in the codebase, as it may in some cases have rights through contractors
- added note about looking for information in individual components
- removed incorrect CC information about Sajax (which appears to be BSD-licensed, according to its file header in skins/common/ajax.js
- rephrased the CC license information to make more clear and precise