Moved vendored js/css into public/vendor and documented sources (#1484) (#2241)

* Cleaning up public/ and documenting js/css libs.

This commit mostly addresses #1484 by moving vendor'ed plugins into a
vendor/ directory and documenting their upstream source and license in
vendor/librejs.html.

This also proves gitea is using only open source js/css libraries which
helps toward reaching #1524.

* Removing unused css file.

The version of this file in use is located at:
  vendor/plugins/highlight/github.css

* Cleaned up librejs.html and added javascript header

A SafeJS function was added to templates/helper.go to allow keeping
comments inside of javascript.

A javascript comment was added in the header of templates/base/head.tmpl
to mark all non-inline source as free.

The librejs.html file was updated to meet the current librejs spec. I
have now verified that the librejs plugin detects most of the scripts
included in gitea and suspect the non-free detections are the result of
a bug in the plugin. I believe this commit is enough to meet the C0.0
requirement of #1534.

* Updating SafeJS function per lint suggestion

* Added VERSIONS file, per request

templates/repo/view_file.tmpl

@@ -48,7 +48,7 @@
 							<strong>{{.i18n.Tr "repo.video_not_supported_in_browser"}}</strong>
 						</video>
 					{{else if .IsPDFFile}}
-						<iframe width="100%" height="600px" src="{{AppSubUrl}}/plugins/pdfjs-1.4.20/web/viewer.html?file={{EscapePound $.RawFileLink}}"></iframe>
+						<iframe width="100%" height="600px" src="{{AppSubUrl}}/vendor/plugins/pdfjs/web/viewer.html?file={{EscapePound $.RawFileLink}}"></iframe>
 					{{else}}
 						<a href="{{EscapePound $.RawFileLink}}" rel="nofollow" class="btn btn-gray btn-radius">{{.i18n.Tr "repo.file_view_raw"}}</a>
 					{{end}}