(bug 35685) Add section with entry point URLs to Special:Version

(bug 35685) Add section with entry point URLs to Special:Version

This'll make it easier for people to figure out where api.php is, and it
should help debug bad settings for $wgArticlePath/$wgScriptPath too.

Patchset 2: address review comments
Patchset 3: rebased to fix conflict in RELEASE-NOTES-1.20
Patchset 4: [Sam] whitespace
Patchset 5: moved down entry points list, fixed mw.org Manual: links
Patchset 6: rebased for another RELEASE-NOTES-1.20 conflict

Change-Id: I86eb84aca38abe214dfa3c3aeb3a9b6e586c8602