Update README & COPYING

Update README & COPYING

This patch contains a re-write of MediaWiki's README. The current README has
been obsoleted by the passage of time, and its current patchwork of contents
is not especially helpful or flattering to MediaWiki. GitHub automatically
renders a repository's README below its file listings; GitHub's popularity
thus means that this file is a lot more visible than it once was and is more
likely to shape a prospective developer's first impression of MediaWiki as a
software platform.

The re-write was conducted on-wiki, at <https://www.mediawiki.org/wiki/README>,
and advertised on wikitech-l. Not much time has passed since the announcement,
but even so the updated README is already considerably better, and there's no
reason not to continue synchronizing changes from the wiki page on an ongoing
basis, so the editorial work on MediaWiki.org need not stop as a consequence of
this change being merged.

This patch also includes an update to COPYING, incorporating legal text that
has been removed from the README, and adding some more information about
licensing. These changes include:

- 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

Bug: 50552
Change-Id: Iff7df2567bd90c136ef81fb8dbe7b6d4408be75f