* 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

* Optimize and fix autolink function (#1442)

* Fix bug in autolink recursive fallback function

Signed-off-by: Andrew Boyarshin <andrew.boyarshin@gmail.com>

* Markdown rendering overhaul

Cleaned up and squashed commits into single one.

Signed-off-by: Andrew Boyarshin <boyarshinand@gmail.com>

* Fix markdown API, add markdown module and API tests, improve code coverage

Signed-off-by: Andrew Boyarshin <boyarshinand@gmail.com>