Move 3rd party js/css into public/vendor and document sources (#2383)

This commit cherry picks the following commits (master -> v1.2):
  a915a09
  874f9be
  233bbcf

Ref: https://github.com/go-gitea/gitea/issues/1484
Signed-off-by: Michael Lustfield <michael@lustfield.net>

public/vendor/plugins/codemirror/.gitattributes

@@ -0,0 +1,8 @@
+*.txt   text
+*.js    text
+*.html  text
+*.md    text
+*.json  text
+*.yml   text
+*.css   text
+*.svg   text