Add .ignore file for search tools (#9417)

* Add .ignore file for search tools

Tools like `rg` [1] and `ag` [2] support a `.ignore` file which defines
files that are ignored during a recursive search. This adds the file
which makes recursive file searches in the code base ignore files that
are generally not desirable to search in like bindata, vendored files
and compiled frontend assets.

[1] https://github.com/BurntSushi/ripgrep/
[2] https://github.com/ggreer/the_silver_searcher

* add leading slash

* remove files already in .gitignore

.ignore

@@ -0,0 +1,5 @@
+/vendor
+/public/vendor
+/modules/options/bindata.go
+/modules/public/bindata.go
+/modules/templates/bindata.go